[C#] Add code-graph extractor

[doctor-ew]

Target branch: main

Language

• Language: C#
• File extensions: .cs
• Tree-sitter grammar repository: https://github.com/tree-sitter/tree-sitter-c-sharp
• Grammar version or commit: tree-sitter-c-sharp@0.23.5 (npm) — note: use this package's own bundled tree-sitter-c_sharp.wasm, not tree-sitter-wasms@0.1.12's copy of the same filename; they are different grammar revisions and several documented fields (variable_declarator.name, using_directive.name) resolve to undefined via childForFieldName against the older build.
• Grammar license: MIT

Scope

A tree-sitter-only extractor (no compiler integration, following the python.ts/rust.ts pattern rather than typescript.ts's compiler-based path) covering the common C# declaration and reference surface: classes, interfaces, structs, enums (with members), methods (incl. constructors/destructors, static, async), properties, fields (incl. const → constant), parameters, namespaces (block and file-scoped, including nesting), using imports, attributes as decorates, calls/instantiates/extends/implements references.

Known, documented limitation: base_list has no field structure and does not syntactically distinguish a base class from an implemented interface (verified against node-types.json). The extractor uses a first-listed-entry-is-the-base-class heuristic (extends for entry 0 on a class/struct, implements for the rest; every entry is extends on an interface) — a documented best-effort, not a semantic guarantee.

Not in scope for this first pass: generics/type-parameter capture (typeParameters), LINQ query-clause references, records/delegates get a best-fit existing NodeKind (class, type_alias) rather than dedicated kinds.

Expected nodes

class, struct, interface, enum, enum_member, method, function (local functions), property, field, constant (const fields), parameter, namespace, type_alias (delegates), file.

Expected edges

contains, calls, imports (using), extends, implements, instantiates (new T()), decorates (attributes).

Required proof

• I have read Extending the code graph
• The language exists in the shared Language vocabulary ("csharp" is already in LANGUAGES, src/graph/types.ts) — no core vocabulary addition needed
• The extractor will use the frozen LanguageExtractor interface
• A compatible grammar WASM file will be vendored and registered
• File extensions and the extractor will be registered
• A representative fixture will be added
• Focused tests will assert node and edge shape
• npm run typecheck, npm test, and npm run build will pass
• No identity, reconciliation, schema, or drift-semantics changes are included

Blocked by

None.

Additional context

Implementation is done and validated: 11 focused tests (extractor-csharp.test.ts) plus a full local npm test run (533/533 passing, no regressions), and a real-world smoke test against a ~640-file C# codebase (a .NET Azure Functions calculation engine) — 694 .cs-tree files, 0 partial/failed, 22,694 nodes / 25,250 edges. Opening a PR against this issue next.