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>

ARCHITECTURE.md

@@ -89,10 +89,9 @@ STPViewer/
 │   ├── run-STPViewer.ps1.example
 │   └── run-STPViewer.sh.example
 │
-├── For_AI/                    # 🚫 gitignored — AI 協作素材
+├── For_AI/                    # 🚫 gitignored — AI 協作素材 + 測試模型 *.stp（客戶料號不入 git）
 │
 ├── STPViewer.sln
-├── *.stp                      # 根目錄現有測試模型（Amphenol connector）
 │
 └── src/
     └── STPViewer/
@@ -335,6 +334,14 @@ NuGet 相依（自動還原）：`CADability`、`HelixToolkit.Wpf`、`CommunityT
 
 **驗收：** 拖 X 箭頭零件僅沿世界 X 移動（與視角無關）；拖環任意角度旋轉；放開後量測/干涉與顯示位置一致
 
+### Phase 11 — Gizmo always-on-top 疊圖層（工作量：M）
+- [x] 操作器移到獨立透明 `Viewport3D`（`gizmoOverlay`）疊在主視窗上 → 不在主場景，**永不被實體遮擋、永遠可抓**
+- [x] `_overlayCamera` 在主相機 `Changed` 時同步（Position/方向/FOV/near-far）；同尺寸 → 投影一致、操作器精準貼合
+- [x] raw Viewport3D 空白處不吃滑鼠 → 穿透回主視窗（orbit/縮放/量測不受影響）；`IsHitTestVisible` 綁 `GizmoEnabled`
+- [x] 放開事件改掛 overlay（`handledEventsToo`）；`_gizmoBakePending` 防同次重複烘焙
+
+**驗收：** 操作器從任何視角都浮在零件上可見可抓；開啟操作器時右鍵 orbit / 量測仍正常（穿透）
+
 ---
 
 ## Future Extensions（下一輪）