In-app shape copy and paste (Ctrl+C / Ctrl+V)

[trailcode]

Summary

• Adds remappable Ctrl+C / Ctrl+V (edit.copy / edit.paste) for an in-app shape clipboard.
• Deep-copies selected solids, or a Shape List group subtree when the selection matches that group's descendant solids.
• Paste inserts under the current group (new ids, uniquified names, same pose) with one Shape_add_delta; clipboard survives New.
• Docs, settings defaults, and unit tests included.

Closes #242

Test plan

• EzyCad_tests --gtest_filter=Shp_test.Copy*:Shp_test.New_file*
• Manual: select solids → Ctrl+C / Ctrl+V (in place, undo)
• Manual: click Shape List group → Ctrl+C → paste preserves nesting
• Manual: copy → New → paste into blank project
• Settings → Keyboard shortcuts shows Copy / Paste (Ctrl+C / Ctrl+V)
• Script console text Ctrl+C/V still works while typing
• Spot-check docs/usage.md / usage-settings.md / CHANGELOG

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Note

Medium Risk
New document mutation path with hierarchy edge cases (group subtree, sibling paste, cycle guards); mitigated by extensive unit tests and all-or-nothing paste validation.

Overview
Adds in-app shape copy/paste via remappable edit.copy / edit.paste (defaults Ctrl+C / Ctrl+V), wired through hotkeys and Occt_view::copy_selected_shapes / paste_clipboard_shapes. Copy snapshots selected solids or, when the selection exactly matches a Shape List group’s descendant solids, the whole subtree into a session Shape_rec forest with independent BREP (not the OS clipboard). Paste deep-copies under the current group (new ids, uniquified names, same pose), uses one Shape_add_delta for undo, and pastes as a sibling when the current group is still the copied group root.

File → New keeps clipboard contents but clears live source-root tracking so copy → New → paste works. Parent-chain walks (would_reparent_create_cycle, shape_descendant_solids, shape_ancestors_visible) and Shape List drawing gain cycle guards to avoid hangs on corrupt hierarchy.

Docs, res/ezycad_settings.json, and Shp_test Copy_* / New_file_keeps_shape_clipboard tests are included. Sketch geometry and OS clipboard are unchanged.

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