Add temporary shape cross-section preview tool

[trailcode]

Summary

Add an interactive Shape cross-section tool that cuts selected solids with a shared local plane and shows a temporary wire preview (no sketch commit in v0).

Problem

• There is no way to inspect a plane cut through solids before implementing sketch-from-shape.
• Section experiments need a shape-local plane (local frame + offset), multi-select behavior, and clear temporary AIS feedback.

Implemented scope

Feature:

• Toolbar mode Shape_cross_section with Options for local XY / XZ / YZ, Invert normal, Hide back side (display-only AIS clip, on by default), bbox-ranged Offset slider, and Clip (undoable half-space BREP cut: keep positive-normal side, replace originals).
• Shared cutting plane for multi-select: orientation from the first selected solid's local axes; origin at the selection bbox center.
• Temporary cyan section wires plus yellow translucent plane fill, outline, and normal arrow (preview AIS not in document / undo; Clip commits document shapes with undo).
• Auto-preview on mode enter (if solids already selected), on selection change, and when plane/offset change. Offset/plane dragging updates the yellow plane immediately; section wires run asynchronously (cancel + latest-pending) so Options stays responsive.
• Desktop: parallel per-solid section preview (worker pool). WASM: serial for that pool (see agents/plans/wasm-multithreading.md).
• Empty selection shows a bold Options prompt to select shapes; clearing selection or leaving the mode removes the preview.
• Solids the plane misses are skipped; preview clears only if nothing intersects.
• Shape local frames (gp_Ax3) baked on move/rotate/scale finalize and persisted in JSON / shape undo records.

Code (high level):

• src/shp_cross_section.h / .cpp
• Mode, Options, and enter-selection restore in gui* / mode.h
• Frame support on Shp + deltas / move-rotate-scale
• Focused tests in tests/shp_tests.cpp

Docs:

• docs/usage.md, docs/usage-settings.md, CHANGELOG.md
• src/doc/gui.md, src/doc/shape.md, agents/plans/cross-section-tool.md

Acceptance criteria

• Select solids then enter the tool (or enter then select): preview updates automatically.
• Changing Local XY/XZ/YZ, Invert normal, or Offset updates the preview.
• Hide back side clips display opposite the yellow arrow (default on).
• Clip half-space cut replaces selected solids (undoable).
• Empty selection shows bold "Select one or more shapes." and clears the preview.
• Esc / leaving the mode clears temporary AIS.
• Focused tests: EzyCad_tests --gtest_filter=Shp_cross_section.*:Shp_test.Cross_section*
• Desktop Release build succeeds; wasm note: shared src/ stays dual-version compatible.

Follow-ups (out of scope)

• Promote section curves to an editable sketch (weld/dedup; fused mid-plane can return extra line edges).
• Optional export / copy of section wires.
• WASM pthread pool for parallel section preview (agents/plans/wasm-multithreading.md).
• WASM Alt + LMB drag multi-select (WASM: Alt + LMB drag does not multi-select shapes #220) — tracked separately; not part of this tool.

Related

• PR: Add temporary shape cross-section preview tool #219
• Plan: agents/plans/cross-section-tool.md
• Draft: agents/drafts/issues/active/gh-218-cross-section-tool.md
• Depends on shape local frames (shipped with this work).
• Related tracking: WASM: Alt + LMB drag does not multi-select shapes #220 (agents/plans/wasm-alt-drag-multiselect.md)