docs: revert record + spin guard — web transparency fix (0f441d8) FAILED and reverted (1295e0e)

User tested: 'didn't work at all, and broke additional crap.' Second failed remedy for
web-source-over-webcam dark composite -> AGENTS.md spin guard: no third guess; next attempt
requires external research (OBS/CEV browser-source transparency in recorded output, cited URL)
plus the blank-page falsifier. HANDOFF OPEN list updated: research-first step added, #11 noted
landed as d35823a.

MyMistakes.md

@@ -15,6 +15,21 @@
 
 ## 🔬 Recipes registry
 
+### ⚠ FAILED remedy (spin guard, 2nd strike) — web overlay composites dark over the webcam
+
+(2026-09-05, takes 19-2/20 + user re-test) Attempt `0f441d8`: feed the compositor the FULL
+transparent-background canvas instead of the alpha crop (crop = measure-only). User verdict:
+**"didn't work at all — broke additional crap."** Reverted (`1295e0e`). I hypothesized the
+alpha-crop→UniformToFill zoom; the fix changed that and the recording was still dark, so the
+crop→zoom was NOT the mechanism. **Due to AGENTS.md's spin guard this is the second failed remedy —
+NO third guess over the same code.** Before the next attempt: research externally how OBS/CEV keep
+browser-source transparency in the *recorded* composite (cite URL), AND test the simplest falsifier
+first — a blank transparent page with no widget content — to decide whether the dark band is the
+widget's own page background (host pages that set their own bg defeat the injection script) rather
+than the compositor. Do not re-derive; the seam candidates (CapturePreviewAsync PNG alpha through
+FormatConvertedBitmap premultiplied/straight; preview-dispatcher vs pump-thread consumers) stay
+unconfirmed and untouched until the research names one of them.
+
 ### Shrink / re-encode an image for the README (screenshots → small hero image)
 
 Worked out 2026-08-29 (the recipe was NEVER recorded the first time it was done, so