feat(ui): Always-on-top gizmo overlay; remove customer STEP files (v0.3.1)

- Gizmo now renders in a separate transparent Viewport3D overlaid on
  the main view, so the manipulator is never occluded inside solid
  parts and is always grabbable from any angle
- Overlay camera follows the main camera (position/dir/FOV/near-far)
  on every change; raw Viewport3D lets empty-area clicks pass through
  so orbit / zoom / measurement still work; IsHitTestVisible bound to
  GizmoEnabled. Mouse-up handled on the overlay (handledEventsToo),
  with _gizmoBakePending guarding against double-bake
- Window title shows version (STPViewer V0.3.1)
- Remove the two test STEP models from the repo (customer part
  numbers) -> moved to For_AI/ (gitignored); add *.stp/*.step to
  .gitignore so CAD files never get committed again
- Docs updated (ARCHITECTURE Phase 11, CLAUDE.md, README)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -13,3 +13,7 @@ secret/*
 
 # AI 協作素材不入 git
 For_AI/
+
+# 客戶 CAD 料號檔不入 git（避免外流）；測試模型放 For_AI/
+*.stp
+*.step