Fix WASM Alt+LMB rectangle multi-select (#220)

[trailcode]

Summary

• Fix WASM Alt+LMB rubber-band multi-select: poll modifiers/cursor via a non-owning GLFWwindow* (Occt_glfw_win is not created on Emscripten, so live Alt was always missing and OCCT rebound the gesture to orbit).
• After OCCT SelectRectangle, also select displayed document solids whose projected AABB intersects the band (complex / STEP parts stock picking often skipped). On WASM, map OCCT window pixels into GLFW cursor space.
• Shape List: selection tint follows AIS only; current group uses a weaker tint so it is not mistaken for selection after Alt-drag.
• Docs: usage.md, usage-occt-view.md, gui.md, CHANGELOG; plan wasm-alt-drag-multiselect.md.

Fixes #220

Test plan

• Desktop: Alt+LMB box-selects multiple shapes (including complex solids under Shape List groups)
• WASM: Alt+LMB multi-select works; click canvas first if the browser steals Alt
• Ctrl+click multi-select still works
• Shape List current-group highlight is distinct from AIS selection
• CI / EzyCad_tests (includes Grouped_solids_stay_displayed_and_selectable)

Related

• Issue: WASM: Alt + LMB drag does not multi-select shapes #220
• Plan: agents/plans/wasm-alt-drag-multiselect.md

---

Note

Medium Risk
Touches core 3D input, selection override, and shared modifier polling on desktop and WASM; behavior change is localized to Alt-drag and post-rectangle selection.

Overview
Fixes #220 by making Alt + left-drag rectangle multi-select work on the WASM build and improving rubber-band picking for complex parts.

WASM input: Occt_view stores a non-owning GLFWwindow* for live modifier and cursor polling (Emscripten never creates Occt_glfw_win, so Alt was always missing on mouse-move and OCCT treated the gesture as orbit). Zoom and mouse paths use the same GLFW cursor space.

Rubber-band picking: After OCCT SelectRectangle, displayed document solids whose projected AABB overlaps the band are added to the selection—addressing misses on complex / STEP BREPs and grouped shapes. On WASM, projected pixels are scaled from OCCT window size to GLFW/canvas size.

Shape List: Row highlight for selection follows the 3D viewer only; the current group gets a lighter tint so it is not confused with multi-select after Alt-drag.

Docs, CHANGELOG, agent plan/drafts, and a grouped-solid selection test are updated.

^{Reviewed by Cursor Bugbot for commit d7e2561. Bugbot is set up for automated code reviews on this repo. Configure here.}