fix(web): re-point the widget diagnostic at the REAL document — the transparency premise gets verified

The one-shot dump + alpha log was bound to the FIRST capture ever = the initial
about:blank placeholder (alpha=0, rgb=0, 5/5 sessions) — a blind instrument. The
pre-parse transparency injection (1a39b09) is intact but was NEVER verified
(MyMistakes '? VERIFY'), and the 2026-09-10 take still shows a black, opaque
block. Stop guessing: NavigationCompleted for a non-about:blank document now
arms WidgetDumpRemaining=5; the next 5 post-paint captures dump
%TEMP%\ytLive-web-<id>-w1..5.png + shared AlphaStats(min/max/mean/zero%) +
FindContentBounds to startup.log.

The stats line alone names the branch: zero%≈opaque ⇒ the injection did not hold
for this widget's CSS (fix: !important stylesheet / chroma-key); large zero% +
tight bounds ⇒ capture IS transparent and the black lives in the compositor
blend. No new test: WebView2 runtime is not instantiable in the suite and the
re-point is log-only; 8/8 WebView2 tests still pass.

ai.md

@@ -911,6 +911,16 @@ Full suite 290/291 passing, the sole failure the pre-existing compositor pixel t
   `NamedPipeAudioWriter.DroppedWrites` — names the exact stage on the next take without new
   code. `AudioMixer.FillAndMix` now returns `(MicRms, MicDrained, LoopDrained)` for the telemetry
   accumulation. Full suite 290/291, same pre-existing sole failure.
+- **Slice 13 (web transparency diagnostic re-point, 2026-09-10):** the "?" in MyMistakes'
+  "RESOLVED (? VERIFY)" is being closed. The one-shot widget dump + alpha stats bound to the
+  FIRST capture ever = the initial about:blank document — a blind instrument. NavigationCompleted
+  for the REAL widget URL now arms `WidgetDumpRemaining = 5`; those 5 post-paint captures dump
+  `%TEMP%\ytLive-web-<id>-w1..5.png` + the shared `AlphaStats(...)` line (alpha[min/max/mean/zero%])
+  + FindContentBounds result to startup.log. That line alone decides the fix branch: zero% ⇒ the
+  pre-parse injection did not hold for this widget (opaque capture → chroma-key / stronger DOM
+  injection); large zero% + tight contentBounds ⇒ the capture IS transparent and the recording's
+  black block lives downstream (compositor blend/underlay). `AlphaStats` extracted as the shared
+  sampler. No new tests (WebView2 runtime not instantiable in tests; the re-point is log-only).
 - **Stop ordering matters:** `StopAsync` stops the encoder — since slice 10 it FLUSHES the pending
   queue (`Channel.TryComplete` → drain writes the leftovers, closes stdin → EOF → ffmpeg finalizes+exits;
   an accepted frame is never lost) — **before** awaiting the loop. The old reverse-order deadlock was