feat(ui): Add drag mode for direct part manipulation (v0.2.0)

- New Drag mode (hand cursor): left-press a part and drag to move
  its whole file along the screen plane; release to commit
- During drag only a temporary TranslateTransform3D is applied
  (GPU-side, smooth even on 430k-triangle assemblies); on release
  the offset is baked into B-rep + meshes + edges via TranslateRoot,
  so measurement accuracy is unaffected
- Merged-mesh hit-testing via _mergedMap (browse rendering active
  in drag mode); edges suspended while dragging
- Mouse capture keeps drag alive outside the window; lost capture
  commits at current position
- Right-button rotate / wheel zoom / Shift+left pan unchanged
- Rename small test model to test.stp; docs updated
  (ARCHITECTURE Phase 9, CLAUDE.md conventions, README)

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

README.md

@@ -21,6 +21,7 @@ STP/STEP 3D 檢視器（Windows 桌面程式，C# .NET 8 WPF）— 多檔匯入
   | ⤚ 對齊 | 點「要移動零件」一點 + 目標點 → 純平移該檔案使兩點貼合（B-rep 整體位移） |
   | 🎯 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
 - **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