[curved] New jts-curved module with SFA / ISO 19125-2 geometry types

Adds an opt-in module that implements the OGC SFA / ISO 19125-2
extended geometry types on top of the extension hooks added to
jts-core in the prior commit.

Geometry types (in org.locationtech.jts.geom.curved)
- CircularString    extends LineString
- CompoundCurve     extends LineString  (member structure collapsed in
                                         this phase)
- CurvePolygon      extends Polygon     (rings linearised on read)
- MultiCurve        extends MultiLineString
- MultiSurface      extends MultiPolygon
- Triangle          extends Polygon     (Polygon with one 4-point ring)
- PolyhedralSurface extends MultiPolygon
- Tin               extends PolyhedralSurface

`Triangle` lives in `.geom.curved` because the existing static-utility
class `org.locationtech.jts.geom.Triangle` (centroid, circumradius)
is preserved unchanged.

Readers / writers (in org.locationtech.jts.io.curved)
- CurvedWKTReader extends WKTReader, overrides
  `readOtherGeometryText` to dispatch the eight new keywords. Composite
  types (CompoundCurve, CurvePolygon, MultiCurve, MultiSurface) iterate
  heterogeneous members via `lookAheadWord`-based peeking. CompoundCurve
  also accepts the writer's flat round-trip form.
- CurvedWKTWriter extends WKTWriter; phase-1 marker. Emission already
  works correctly via the core `getGeometryType().toUpperCase()` change
  routed through the existing `instanceof` ladder.

Tests
- 54 round-trip tests across 8 files covering XY / XYZ / XYM / XYZM /
  EMPTY / round-trip / structural-validation-deferred negative cases.
- All pass against the new module; full jts-core suite (2282 tests)
  still passes unchanged.

Out of scope (deferred to later phases per the proposal)
- Validation (Triangle 4-point rule, CircularString odd point count,
  CompoundCurve member connectivity, Tin triangle-only patches).
- Member-structured CompoundCurve / CurvePolygon emission (currently
  collapsed to flat coordinates / linear rings).
- WKBReader / WKBWriter for SFA-MM type codes.
- Native curve-aware spatial operations (intersects, contains, area,
  buffer). Curves currently fall through to their parent type's
  polyline / polygon behaviour.
- copy() returning the correct subclass.
- JTSTestBuilder UI integration.

References
- Discussion: locationtech/jts#1193
- Design template: NetTopologySuite/NetTopologySuite#526

Author: grootstebozewolf

modules/curved/pom.xml

@@ -0,0 +1,64 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>org.locationtech.jts</groupId>
+        <artifactId>jts-modules</artifactId>
+        <version>1.20.1-SNAPSHOT</version>
+    </parent>
+    <groupId>org.locationtech.jts</groupId>
+    <artifactId>jts-curved</artifactId>
+    <name>${project.groupId}:${project.artifactId}</name>
+    <packaging>jar</packaging>
+
+    <description>
+        OGC SFA / ISO 19125-2 extended geometry types (CircularString,
+        CompoundCurve, CurvePolygon, MultiCurve, MultiSurface, Triangle,
+        PolyhedralSurface, Tin) and the corresponding WKT readers/writers.
+        Built on top of the extension hooks in jts-core; opt-in.
+    </description>
+
+    <build>
+        <plugins>
+            <plugin>
+                <artifactId>maven-jar-plugin</artifactId>
+                <configuration>
+                  <archive>
+                    <manifestEntries>
+                      <Automatic-Module-Name>org.locationtech.jts.curved</Automatic-Module-Name>
+                    </manifestEntries>
+                    <manifestSections>
+                      <manifestSection>
+                        <name>org.locationtech.jts.curved</name>
+                        <manifestEntries>
+                          <Sealed>true</Sealed>
+                        </manifestEntries>
+                      </manifestSection>
+                    </manifestSections>
+                  </archive>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.locationtech.jts</groupId>
+            <artifactId>jts-core</artifactId>
+            <version>${project.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.locationtech.jts</groupId>
+            <artifactId>jts-core</artifactId>
+            <version>${project.version}</version>
+            <classifier>tests</classifier>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+</project>

modules/curved/src/main/java/org/locationtech/jts/geom/curved/CircularString.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2026 grootstebozewolf
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * and Eclipse Distribution License v. 1.0 which accompanies this distribution.
+ * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
+ * and the Eclipse Distribution License is available at
+ *
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ */
+package org.locationtech.jts.geom.curved;
+
+import org.locationtech.jts.geom.CoordinateSequence;
+import org.locationtech.jts.geom.GeometryFactory;
+import org.locationtech.jts.geom.LineString;
+
+/**
+ * A connected sequence of circular arcs, where each consecutive triple of
+ * control points (start, mid, end) defines one arc and the end point of one
+ * arc is the start point of the next.
+ * <p>
+ * This is a phase-1 stand-in: the control points are stored as a single
+ * {@link CoordinateSequence} (inherited via {@link LineString}) and spatial
+ * operations fall through to the parent's polyline behaviour. Native
+ * arc-aware algorithms are out of scope for this module today.
+ */
+public class CircularString extends LineString {
+  private static final long serialVersionUID = 1L;
+
+  public CircularString(CoordinateSequence points, GeometryFactory factory) {
+    super(points, factory);
+  }
+
+  @Override
+  public String getGeometryType() {
+    return "CircularString";
+  }
+}

modules/curved/src/main/java/org/locationtech/jts/geom/curved/CompoundCurve.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2026 grootstebozewolf
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * and Eclipse Distribution License v. 1.0 which accompanies this distribution.
+ * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
+ * and the Eclipse Distribution License is available at
+ *
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ */
+package org.locationtech.jts.geom.curved;
+
+import org.locationtech.jts.geom.CoordinateSequence;
+import org.locationtech.jts.geom.GeometryFactory;
+import org.locationtech.jts.geom.LineString;
+
+/**
+ * A connected sequence of {@link LineString} and {@link CircularString}
+ * segments. Phase-1 stand-in: member structure is collapsed to a flat
+ * concatenation of control points. A future phase will preserve segments.
+ */
+public class CompoundCurve extends LineString {
+  private static final long serialVersionUID = 1L;
+
+  public CompoundCurve(CoordinateSequence points, GeometryFactory factory) {
+    super(points, factory);
+  }
+
+  @Override
+  public String getGeometryType() {
+    return "CompoundCurve";
+  }
+}

modules/curved/src/main/java/org/locationtech/jts/geom/curved/CurvePolygon.java

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2026 grootstebozewolf
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * and Eclipse Distribution License v. 1.0 which accompanies this distribution.
+ * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
+ * and the Eclipse Distribution License is available at
+ *
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ */
+package org.locationtech.jts.geom.curved;
+
+import org.locationtech.jts.geom.GeometryFactory;
+import org.locationtech.jts.geom.LinearRing;
+import org.locationtech.jts.geom.Polygon;
+
+/**
+ * A polygon whose rings may be straight, circular, or compound curves.
+ * Phase-1 stand-in: rings are linearised to {@link LinearRing}s on read.
+ */
+public class CurvePolygon extends Polygon {
+  private static final long serialVersionUID = 1L;
+
+  public CurvePolygon(LinearRing shell, LinearRing[] holes, GeometryFactory factory) {
+    super(shell, holes, factory);
+  }
+
+  public CurvePolygon(GeometryFactory factory) {
+    super(null, null, factory);
+  }
+
+  @Override
+  public String getGeometryType() {
+    return "CurvePolygon";
+  }
+}

modules/curved/src/main/java/org/locationtech/jts/geom/curved/MultiCurve.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2026 grootstebozewolf
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * and Eclipse Distribution License v. 1.0 which accompanies this distribution.
+ * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
+ * and the Eclipse Distribution License is available at
+ *
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ */
+package org.locationtech.jts.geom.curved;
+
+import org.locationtech.jts.geom.GeometryFactory;
+import org.locationtech.jts.geom.LineString;
+import org.locationtech.jts.geom.MultiLineString;
+
+/**
+ * A collection of {@link LineString}, {@link CircularString} and
+ * {@link CompoundCurve} members.
+ */
+public class MultiCurve extends MultiLineString {
+  private static final long serialVersionUID = 1L;
+
+  public MultiCurve(LineString[] members, GeometryFactory factory) {
+    super(members, factory);
+  }
+
+  @Override
+  public String getGeometryType() {
+    return "MultiCurve";
+  }
+}

modules/curved/src/main/java/org/locationtech/jts/geom/curved/MultiSurface.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2026 grootstebozewolf
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * and Eclipse Distribution License v. 1.0 which accompanies this distribution.
+ * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
+ * and the Eclipse Distribution License is available at
+ *
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ */
+package org.locationtech.jts.geom.curved;
+
+import org.locationtech.jts.geom.GeometryFactory;
+import org.locationtech.jts.geom.MultiPolygon;
+import org.locationtech.jts.geom.Polygon;
+
+/** A collection of {@link Polygon} and {@link CurvePolygon} members. */
+public class MultiSurface extends MultiPolygon {
+  private static final long serialVersionUID = 1L;
+
+  public MultiSurface(Polygon[] members, GeometryFactory factory) {
+    super(members, factory);
+  }
+
+  @Override
+  public String getGeometryType() {
+    return "MultiSurface";
+  }
+}

modules/curved/src/main/java/org/locationtech/jts/geom/curved/PolyhedralSurface.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2026 grootstebozewolf
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * and Eclipse Distribution License v. 1.0 which accompanies this distribution.
+ * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
+ * and the Eclipse Distribution License is available at
+ *
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ */
+package org.locationtech.jts.geom.curved;
+
+import org.locationtech.jts.geom.GeometryFactory;
+import org.locationtech.jts.geom.MultiPolygon;
+import org.locationtech.jts.geom.Polygon;
+
+/** A contiguous collection of polygonal patches that share edges. */
+public class PolyhedralSurface extends MultiPolygon {
+  private static final long serialVersionUID = 1L;
+
+  public PolyhedralSurface(Polygon[] patches, GeometryFactory factory) {
+    super(patches, factory);
+  }
+
+  @Override
+  public String getGeometryType() {
+    return "PolyhedralSurface";
+  }
+}

modules/curved/src/main/java/org/locationtech/jts/geom/curved/Tin.java

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2026 grootstebozewolf
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * and Eclipse Distribution License v. 1.0 which accompanies this distribution.
+ * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
+ * and the Eclipse Distribution License is available at
+ *
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ */
+package org.locationtech.jts.geom.curved;
+
+import org.locationtech.jts.geom.GeometryFactory;
+import org.locationtech.jts.geom.Polygon;
+
+/** A {@link PolyhedralSurface} whose patches are all triangles (TIN). */
+public class Tin extends PolyhedralSurface {
+  private static final long serialVersionUID = 1L;
+
+  public Tin(Polygon[] patches, GeometryFactory factory) {
+    super(patches, factory);
+  }
+
+  @Override
+  public String getGeometryType() {
+    return "Tin";
+  }
+}

modules/curved/src/main/java/org/locationtech/jts/geom/curved/Triangle.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2026 grootstebozewolf
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * and Eclipse Distribution License v. 1.0 which accompanies this distribution.
+ * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v20.html
+ * and the Eclipse Distribution License is available at
+ *
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ */
+package org.locationtech.jts.geom.curved;
+
+import org.locationtech.jts.geom.GeometryFactory;
+import org.locationtech.jts.geom.LinearRing;
+import org.locationtech.jts.geom.Polygon;
+
+/**
+ * A planar triangle as defined by OGC SFA / ISO 19125-2: a {@link Polygon}
+ * with a single 4-point closed exterior ring and no holes.
+ * <p>
+ * Note: the unrelated static-utility class
+ * {@code org.locationtech.jts.geom.Triangle} (centroid, circumradius, etc.)
+ * is preserved unchanged in jts-core. The geometry type lives here in the
+ * curved package to avoid that name collision.
+ */
+public class Triangle extends Polygon {
+  private static final long serialVersionUID = 1L;
+
+  public Triangle(LinearRing shell, GeometryFactory factory) {
+    super(shell, null, factory);
+  }
+
+  public Triangle(GeometryFactory factory) {
+    super(null, null, factory);
+  }
+
+  @Override
+  public String getGeometryType() {
+    return "Triangle";
+  }
+}