feat(ui): input-event trio — mouse modifiers, canvas drag capture, wheel seam (#567 #568 #569)

[InauguralPhysicist]

First-consumer findings from the DeslanStudio milestone-4c arrangement timeline (#561–#566 series continuation).

#568 — mouse events carry modifier state (foundation)

gfx_poll (src/ext_gfx.c) now attaches shift/ctrl/alt (0/1) to mousemove, mousedown, mouseup, and wheel events via SDL_GetModState() (loaded as an optional symbol; missing symbol reads as no modifiers), mirroring exactly what key events already exposed from keysym.mod. The mask encoding is shared in one poll_set_mods helper. Additive dict fields only — no signature changes; headless tests that synthesize event dicts are unaffected (absent keys read null).

#567 — canvas drag/mousemove seam

ui.dispatch's drag-in-progress path is now registry-driven instead of the hardcoded slider-family type chain:

• Each built-in draggable type (slider, vslider, knob, scrollbar, waveform_view, splitter, color_picker) registers an on_drag(w, ev) handler in its own widget file; dispatch just consults the registry.
• New public API claim_drag of w / release_drag of null: a mousedown handler (e.g. a canvas on_mouse) claims the pointer, and the widget then receives every drag mousemove and the terminating mouseup through its on_mouse(w, ev) callback — press-move-release gestures complete on a canvas.
• Hover mousemoves (no capture) are also delivered to any widget exposing on_mouse, with the full event dict (modifiers included). Branch on ev.type.
• The sharp edge is fixed explicitly: a claimant outside the old type list no longer falls into _update_drag's type_mismatch; a claimant with no handler at all is a silent no-op.

#569 — wheel seam + scroll_panel _ax staleness

• A widget under the cursor exposing on_wheel(w, ev) consumes wheel input before the _find_scrollable_at walk (ev.x/ev.y are the scroll deltas; gfx/lib/ui: mouse events carry no modifier state — shift/ctrl-click semantics impossible #568 modifiers ride along for ctrl+wheel zoom / shift+wheel horizontal). Unconsumed wheel input falls through to the scroll_panel path unchanged.
• The _ax staleness note reproduced: _layout cached scroll_panel children's _ax/_ay without the scroll offset while render and hit-test subtract it. _layout now subtracts scroll_x/scroll_y, so a canvas inside a scrolled panel keeps correct mouse math (cache refreshes each frame's layout pass, as before).

Back-compat

• All seven built-in draggable types keep byte-identical drag math (handlers moved, not changed) — covered by new dispatch-level regression tests for each.
• Existing canvas consumers (ui_demo, deslan_shell, debugger examples) now also receive hover mousemoves in on_mouse; the debugger already branches on ev.type, the other two just update status text. No consumer breaks.
• Event dicts gained fields only; dispatch still accepts modifier-less synthesized events.
• Wheel behavior without on_wheel is unchanged.

Tests / demo / docs

• UI suite [63] extended 81 → 115 checks: canvas hover + full gesture capture (the lib/ui: canvas on_mouse never receives mousemove — no drag interactions possible on a custom-painted widget #567 repro), modifier passthrough, the handler-less-claimant sharp edge, per-type drag back-compat (all seven types via dispatch), wheel consume + fallback + modifier pairing, scroll-offset _ax/_ay cache.
• New minimal demo examples/ui_canvas_events.eigs: timeline-style canvas with clip drag-move, rubber-band select, ctrl+click selection toggle, wheel pan, ctrl+wheel zoom-at-cursor — all three seams in ~200 lines.
• Docs: BUILTINS.md gfx_poll row, STDLIB.md UI-toolkit seams paragraph, lib/ui.eigs header, canvas constructor doc, CHANGELOG.

Gates

• make test release: 2837/2837
• make asan + ASAN_OPTIONS=detect_leaks=1 full suite: 2841/2841, leak tally 0
• make jit-smoke: pass
• make gfx: builds clean (and the full suite passes against the gfx binary)
• make freestanding-check: pass (gfx not in the freestanding surface)

Closes #567
Closes #568
Closes #569

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