Docs: record the zoom box as TASK 52 and fix the task-numbering drift (Unit: zoom docs)

The zoom feature shipped without a task record while its index rows carried the
"TASK 51" label — which belongs to the webcam background effects (Remove + Blur).
For an honest catalogue and handoff:

- TASKS/task-52-zoom-box.md: feature provenance (creator-requested), the three
  2026-10-08 corrections (loupe -> anchored capture over the full composite drawn
  last -> never show the dim shading), shapes, implementation map, tests, DONE.
- TASKS.md: row 52 added (shipped); row 51 reworded — unblocked now that the
  zoom build shipped.
- TASK 51 -> TASK 52 in Models/index.md, ViewModels/index.md, and the two code
  comments that pointed at the zoom as "TASK 51".
- HANDOFF.md: pre-push readiness notes (5 commits ahead, push-safe).

ViewModels/MainViewModel.cs

@@ -85,7 +85,7 @@ public partial class MainViewModel : ViewModelBase
     public bool ShowZoomOverlay => _zoomState.IsActive;
 
     // Zoom activation/move/dismiss and the zoom frame builder live in
-    // MainViewModel.Zoom.cs — the zoom box is a real composited layer (see TASK 51).
+    // MainViewModel.Zoom.cs — the zoom box is a real composited layer (see TASK 52).
 
     private void RebuildQualityDimRects()
     {