perf: Render merged mesh in measure mode to fix large-assembly orbit lag (v0.3.2)

Measurement modes used to render per-face content (one GeometryModel3D per
B-rep face). On a large assembly (64k faces measured) that is 64k draw calls,
and WPF re-walks the whole visual tree every frame, so orbiting in a
measurement tool was severely laggy while plain browse mode (which renders the
merged mesh, one model per file) stayed smooth.

Render the merged mesh whenever not sectioning (browse AND measure), and
resolve the picked B-rep face from the hit triangle's vertex index via
_mergedFaceRanges + ResolveMergedFace (binary search over per-face vertex-start
boundaries recorded at import — BuildMergedMesh appends faces in order with no
vertex welding, so the ranges are exact and stay valid across translation).
Per-face FacesContent is now rendered only in section mode (needs cut
geometry). WPF 3D hit testing is geometric and does not cull back faces, so
single-sided merged meshes still pick hole inner walls.

Also subscribe camera-interaction suspension to HelixViewport3D.CameraChanged
(control-level routed event) in addition to Camera.Changed, so it can't be
orphaned if Helix replaces the camera instance.

Verified: orbit in measure mode is now as smooth as browse; circle/face/edge/
angle/face-distance and section measurements unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ARCHITECTURE.md

@@ -65,8 +65,11 @@ STEP（`.stp` / `.step`）檔案做確認與量測，不需要安裝 SolidWorks
 
 資料流：`*.stp → CADability B-rep → 三角網格(渲染) + B-rep 參照(量測) → Helix Viewport`
 
-每個 Face 對應一個 `GeometryModel3D`，並登錄到 `Dictionary<GeometryModel3D, FaceInfo>`，
-HitTest 命中後可反查回 B-rep Face / Edge 做精確量測。
+每個 Face 對應一個 `GeometryModel3D`（`FacesContent`，剖面模式渲染用），並登錄到 `_faceMap` 反查 B-rep Face / Edge。
+
+效能：非剖面（瀏覽＋量測）渲染**整零件合併網格**（`MergedContent`，每檔 1 個 model），量測 HitTest 改打合併網格、
+用命中三角形的頂點 index 經 `_mergedFaceRanges` 二分搜尋（`ResolveMergedFace`）反查回是哪個面 —
+量測模式不再掛數萬個逐面 model，大組件量測下轉動視角與瀏覽同樣流暢（v0.3.2）。
 
 ---
 
@@ -336,6 +339,24 @@ NuGet 相依（自動還原）：`CADability`、`HelixToolkit.Wpf`、`CommunityT
 
 ---
 
+## Development Phases — 第四輪（效能，全部完成）
+
+### Phase 12 — 大組件量測流暢度（工作量：M）
+
+**問題：** 大檔（39MB / 64k 面）瀏覽轉動順，但一進量測模式轉動就嚴重卡頓。
+**根因：** 量測模式渲染逐面（每個 B-rep 面 1 個 `GeometryModel3D`），64k 面 = 64k draw call，
+WPF retained-mode 每幀重走 visual tree → frame rate 崩。瀏覽模式因渲染合併網格（每檔 1 個 model）所以順。
+
+- [x] 量測拾取改打**合併網格** + 三角形頂點 index 反查面（`_mergedFaceRanges` + `ResolveMergedFace` 二分搜尋）
+- [x] `ApplyRenderMode()` 改為：非剖面（瀏覽＋量測）→ 合併網格；剖面 → 逐面。量測模式不再掛數萬個逐面 model
+- [x] `BuildMergedMesh` 依面序串接（無焊接共用頂點），匯入時同步記錄每面頂點起始邊界 + `FaceInfo`
+- [x] 相機暫停事件改訂 `HelixViewport3D.CameraChanged`（控制項層級）＋ `Camera.Changed` 保底，避免相機實例被換掉時訂閱孤兒化
+- [x] 驗證：WPF 3D hit-test 純幾何不剔背面 → 打單面合併網格仍命中孔內壁；圓/面/邊/角度/面距/剖面量測數值與逐面一致
+
+**驗收：** 大組件量測模式下轉動視角與瀏覽同樣流暢；圓孔/面/邊/角度/面距量測精度不變；剖面模式量測仍正常
+
+---
+
 ## Future Extensions（下一輪）
 
 - 剖切面封口（cap）填實（目前剖開處可見內部背面材質）