docs: backlog — the keystroke swallow is fixed and refuted as the cause

docs/re/BACKLOG.md

@@ -125,9 +125,22 @@ See [`structures/ui-composable-bundles.md`](structures/ui-composable-bundles.md)
   Two harness bugs were found and fixed on the way (`--audio` is not a cvar and
   broke eight boot scripts; `vgamepad` no longer exists but `skip_intro.sh`
   still called it, pressing nothing while exiting 0). Neither unblocks the title.
-  Next step: boot with the emulator's stdout **kept** and check the
-  `[RE-INPUT] XamInputGetKeystrokeEx swallowed by IsUIActive` log already in the
-  canary branch. See
+  🔬 **2026-08-19, second attempt: a root cause found and then refuted.** With
+  the emulator's stdout kept, the log said it at once — `dialogs=1`, and
+  **8 388 601** `XamInputGetKeystrokeEx` calls discarded by `IsUIActive()`. The
+  dialog is a **sign-in UI opened because the requested profile does not exist**:
+  the script asked for `E0300000EFBEA3D4` and only `B13EBABEBABEBABE` is on disc.
+  Signing in the real profile takes the swallow count to **0** — and the title
+  *still* does not advance on a single Ⓐ. Real bug, not the explanation.
+  Four harness bugs fixed on the way (stdout to `/dev/null`; a title probe using
+  a 1280×720 pixel against the 1279×675 game surface, which caused a 600 s
+  timeout with the title on screen; tapping through the movies, which reaches a
+  title that accepts nothing; a hard-coded XUID). Boots now reach the title
+  reliably at ~240 s.
+  **Next probe:** `--log_mask=13` records no kernel *calls*, so it is not known
+  whether the game polls at the title at all. Add a log on the **successful**
+  `XamInputGetKeystrokeEx` return, mirroring the one already on the swallow path;
+  one boot answers it. See
   [`ui-paint-order-third-permutation.md`](ui-paint-order-third-permutation.md).
 * ❔ **Blend mode.** Everything is straight alpha-over. The near-white flash
   quads (`0xf0ffffff`) and coloured ones (`0x60ff0000`) may be additive. The