Revert "fix(web): composite consumes the FULL canvas, alpha crop is measure-only — transparent web margins reveal the webcam again"

This reverts commit 0f441d825a.

ai.md

@@ -27,15 +27,6 @@ This project's hard lesson: the web-source bounding saga (2026-08) burned 9
 commits rediscovering that a browser source is a *fixed canvas* — OBS keeps it at
 a stable page size, crops/hugs content via the box, and clips at the edge. The
 apparent "gap" that ended the saga was a widget's own CSS glow effect, not a bug.
-Refined 2026-09-05 (take-19-2/take-20 "webcam black square"): **measure with the
-alpha crop, but CONSUME the full canvas.** The alpha bounding box (`FindContentBounds`)
-is the widget's true extent for the UI/selection box; the frame handed to the
-compositor must be the FULL transparent-background canvas, because the element rect
-is a viewport onto the page — `UniformToFill` on the CROP zoomed the opaque content
-box to cover the whole element rect (a hard box over the webcam) the moment the
-widget drew any content. Stateful take-1-good/take-2-bad: an idle transparent page
-produces a full-frame crop (≈ the correct full-canvas viewport), any real content
-produced a tight crop + zoom.
 
 **Derived-solution rule (2026-08-29, driven by the image-shrink incident):** when
 you work out any one-off, reusable solution (recipe / workaround / how-to), write