feat(ui): Add gizmo manipulator for axis move/rotate (v0.3.0)

- New "操作器" toggle: select a file in the tree and a 3-axis gizmo
  appears at its center -- XYZ arrows (TranslateManipulator) to move
  along an axis regardless of view angle, and rotation rings
  (RotateManipulator) to rotate any angle about an axis
- Manipulators bind to a proxy ModelVisual3D; the proxy's transform
  is mirrored to all target BodyVisuals during drag (GPU-side, temp)
- On release the accumulated transform is baked into B-rep via
  TransformRoot (deferred with Dispatcher.BeginInvoke to avoid
  reentrancy with the manipulator's own mouse-up handling); the
  gizmo re-centers on the new bounds
- Temporary transforms are cleared with Transform3D.Identity, never
  null (HelixToolkit GetTransform crashes on null child.Transform)
- Selecting another file retargets the gizmo; removing the target
  hides it
- Docs updated (ARCHITECTURE Phase 10, CLAUDE.md, README)

Known limitation: gizmo handles are occluded inside solid parts
(WPF single depth buffer). Next: always-on-top overlay viewport.

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

README.md

@@ -22,6 +22,7 @@ STP/STEP 3D 檢視器（Windows 桌面程式，C# .NET 8 WPF）— 多檔匯入
   | 🎯 三點 | 來源檔 3 特徵點 + 目標檔 3 對應點 → 旋轉+平移一次貼合（方向不同也能對） |
 - **旋轉**：樹面板選檔案 + 工具列 ↻X/↻Y/↻Z 繞中心 +90°（擺正方向用，連按累加）
 - **拖曳** 🖐：手形游標模式，左鍵按住零件沿螢幕平面拖動、放開定位（粗定位用；精確貼合用對齊）；右鍵轉視角不受影響
+- **操作器** ⊹：樹面板選檔案 → 顯示 XYZ 三色箭頭 + 旋轉環（Fusion 360 風格）；拖箭頭沿該軸移動（與視角無關，適合沿插合軸推進）、拖環繞軸轉任意角度，放開即定位
 - **干涉檢查** 🧩：勾選剛好 2 個可見檔案 → 相交時顯示紅色干涉交線 + 相交三角形對數；無相交時回報最小間隙 gap（gap≈0 即為配合 match，共面貼合不算干涉）
 - **剖面**：✂ 開關 + X/Y/Z 軸 + 位置滑桿 + 反向；CPU 網格裁切，原始幾何保留（量測不受影響）
 - **單位**：mm ⇄ inch 一鍵切換，既有量測（清單與 3D 標籤）即時換算