Web source: crop to the frame's ALPHA bounding box — the widget's true rendered extent, no DOM timing, can't break the render

- FindContentBounds scans the Bgra32 capture for the bounding box of non-transparent pixels
  (background is injected transparent, so alpha = widget extent). Crop to that rect + Stretch=Fill
  maps the widget flush under the selection box: full-canvas widgets fall through to the full frame
  (identical to the confirmed-good image); floating widgets get tight-fitted with no dead space.
- Removes QueryContentBoundsAsync + the JS script entirely — the DOM-union attempt (a5b9952) broke
  rendering by measuring an unsettled layout on NavigationCompleted; alpha measurement is immune.
- 3 new unit tests: tight box, no-alpha no-crop, full-alpha no-crop. 7/7 pass, 0 warnings.

TASKS.md

@@ -917,14 +917,16 @@ capturing the primary display (which causes the infinity mirror effect).
 8. ✅ Preview shows live web content (not just a blank rectangle)
 9. ☐ Handle navigation errors, invalid URIs, timeout gracefully
 
-**Rendering model (2026-08-28, canvas-size viewport, NO crop — the only creator-verified state):**
+**Rendering model (2026-08-28, canvas-size viewport + ALPHA-BBOX crop):**
 the page renders at the MASTER CANVAS size (1920×1080), stable, never tracked (no reflow/truncation;
-scrollbars suppressed via `overflow:hidden`). Capture is the FULL frame; `Stretch="Fill"` maps it
-under the box. Image rendering verified good ("renders and resizes beautifully"). WEB BOX DEAD SPACE
-STILL OPEN: previous cropping attempts all failed (JSON double-encode silently disabled the crop;
-scrollWidth returns the full canvas for full-canvas wrappers; content-union measured an unsettled
-layout on NavigationCompleted and BROKE the image). Any further crop needs creator approval first.
-⚠️ `ExecuteScriptAsync` JSON-encodes the returned value — never return `JSON.stringify` from script.
+scrollbars suppressed via `overflow:hidden`). `FindContentBounds` scans the Bgra32 capture and crops
+to the bounding box of non-transparent pixels — the widget's true rendered extent, measured from the
+frame itself (NO DOM query, immune to layout timing; can never truncate content; full-canvas widgets
+fall through to the full frame = prior verified-good image). `Stretch="Fill"` maps the cropped frame
+flush under the selection box → box hugs the widget on all four sides. `QueryContentBoundsAsync` +
+the JS content-bounds script are DELETED. ⚠️ Lesson bank: never return `JSON.stringify` from
+`ExecuteScriptAsync` (double-encodes); never measure DOM stuff on NavigationCompleted (unsettled
+layout broke the image — `a5b9952`); measure rendered pixels instead.
 
 **Properties panel (2026-08-28):** web URI ✓/✕ icon buttons are `IsTabStop="False"` so Tab flows
 X→Y→W→H→URI; the ✕ button now clears the URI textbox (was reverting to the pre-accept snapshot).