Cross-section follow-ups: outline, Clip discard, sketch import, toast log

[trailcode]

Summary

• Optional Show section outline (default off); toast messages also go to the Log.
• Clip removes solids fully on the discarded side instead of erroring; survivors stay clipped replacements (undoable).
• Cross section sketch imports cached section line/circle edges into a new sketch on the cutting plane (undoable).
• Docs, CHANGELOG, and focused tests updated.

Test plan

• Desktop Release build
• EzyCad_tests --gtest_filter=Shp_cross_section.*:Shp_test.Cross_section*
• Manual: enter tool / plane / offset / Hide back / Show section outline / Clip multi-select with a fully discarded solid / Cross section sketch / Esc
• Confirm toast text appears in Log; ezy.msg too
• Docs sync checklist (usage.md, usage-settings.md, CHANGELOG)

Closes #221

Related: #218 #219

---

Note

Medium Risk
Changes touch modeling (Clip deletes geometry, sketch import from OCCT section) and global status logging; behavior is covered by new tests but Clip/sketch paths affect user documents.

Overview
Follow-ups on the Shape cross-section tool and status messaging.

Cross-section: Show section outline (default off) toggles cyan intersection wires from a cached compound without recomputing. Clip no longer errors when a solid is fully on the discarded side—it deletes those inputs with no replacement and reports counts; survivors still become clipped replacements (undoable). Cross section sketch waits for section work if needed, builds a sketch on the cutting plane, imports line and circle edges (full circles split into two semicircles), runs rebuild_faces(), commits undo, and switches to sketch inspection; other curve types are skipped.

Status / Log: GUI::show_message and ezy.msg now append the same text to the Log window. show_error_dialog logs title: message once and shows only the title in the toast to avoid duplicate log lines.

Docs, CHANGELOG, agent plan status, and unit tests cover outline toggle, full discard on clip, and box midplane sketch import.

^{Reviewed by Cursor Bugbot for commit 18b5c0d. Bugbot is set up for automated code reviews on this repo. Configure here.}