feat(fresh-ui): give the wheel an axis

[sinelaw]

Base of a two-PR stack. #3028 (the fresh-editor UI migration) is stacked on top of this.

Why

The scroll model has always been two-dimensional — a viewport's offset and its maximum are both Points, and ViewportProps::scroll is a pair — but nothing could address the horizontal one. Input::Wheel carried a bare scalar and scroll_chain only ever moved y. The axis was implied by the geometry everywhere except where it could be driven.

This surfaced while writing the editor-side input adapter for the migration: translating a horizontal wheel had no target, so the adapter had to decline it and leave it on the legacy path. Rather than encode that gap as an editor workaround — which is the class of thing the migration exists to remove — it is closed here, in the library, before anything depends on it.

What changed

• Axis::{Vertical, Horizontal}, carried on Input::Wheel and on the Event a Wheel listener receives, so a widget that scrolls itself can tell the two apart instead of guessing.
• scroll_chain moves and clamps the axis it was given.
• The wheel travels through propagation as one parameter (Wheel { delta, axis }) rather than two, so the gestures that are not wheels pass Wheel::NONE instead of a bare 0 whose meaning had to be inferred at each call site.

What this deliberately does not do

It does not make the built-in Viewport scroll sideways. A viewport lays its child out under Constraints::new(0, w, ..), bounding it to the window's own width, so content can never overflow horizontally and that maximum stays zero — Viewport is a vertical scroller by construction, and remains one here.

A horizontally scrollable viewport is a separate feature, and nothing needs it yet: a host leaf that scrolls its own content (an editor buffer) does that itself and only needs the axis to reach its handler intact. a_viewport_has_no_horizontal_extent_to_scroll pins that boundary, so if someone later builds a horizontal viewport, the test says what changed.

Verification

tests/pointer.rs covers all three cases: a Wheel listener sees the axis; a vertical wheel still moves only y; a horizontal wheel over a viewport correctly does nothing rather than scrolling the wrong way.

Full suite green — 150 tests, goldens included, so no rendered output moved. The crate still builds standalone with unicode-width as its only runtime dependency.

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Generated by Claude Code