Add temporary shape cross-section preview tool

[trailcode]

Summary

• Adds a temporary Shape cross-section tool for selected solids (cyan section wires + yellow plane/normal annotation; not committed to the document).
• Shared multi-select plane: first-selected local axes, selection bbox center, local XY/XZ/YZ + bbox-ranged offset; Invert normal; Hide back side (AIS clip, on by default); Clip (undoable half-space BREP cut).
• Auto-updates preview on mode enter (pre-selection), selection changes, and plane/offset changes; bold Options prompt when nothing is selected. Offset/plane dragging updates the yellow plane immediately and computes section wires asynchronously so the slider stays responsive.
• Desktop: parallel per-solid section preview; WASM stays serial for that pool. Plan notes for future WASM pthreads: agents/plans/wasm-multithreading.md.
• Adds shape local frames (gp_Ax3) persisted with shapes / undo, updated on move/rotate/scale finalize.

Closes #218

Test plan

• Desktop Release build (EzyCad + EzyCad_tests)
• EzyCad_tests --gtest_filter=Shp_cross_section.*:Shp_test.Cross_section*
• Manual: select solid(s) then click cross-section — preview appears
• Manual: change Local XY/XZ/YZ, Invert normal, Offset — preview updates; Hide back side clips display
• Manual: Clip — originals replaced with half-space result; undo works
• Manual: clear selection — preview clears; bold "Select one or more shapes."
• Manual: Esc / leave mode — temporary AIS cleared
• Docs: usage.md / usage-settings.md / CHANGELOG match behavior

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