Round 24: video probe failing stage; instrument bind + probe steps/error paths

RESEARCH.md

@@ -922,3 +922,41 @@ painted. Root cause found in the disassembly, not guessed:
 
 blue → 3 blinks → 5 s hold → green band (board_init_f) → orange/blue/
 red bands → cyan/white (video probe) → banner.
+
+## Round 24 — C environment alive; video probe is the failing stage
+
+Round 23 (`18cf3e93…`): blue → blinks → hold → bands. The user
+reported (top to bottom): two red-ish stripes, yellow, green — i.e.
+board_init_f (green), board_early_init_f (orange, reads red-ish),
+board_early_init_r (red), misc_init_r (yellow) ALL ran; dram_init
+blue is at the very bottom (y=1820, easy to miss). CRITICAL: the
+screen was NOT cleared and no cyan/white band appeared → the video
+uclass post_probe (whose FIRST action after colour setup is
+video_clear) never ran → the video driver's probe FAILED or the
+device never bound. Everything else (banner needs the vidconsole)
+follows from that. Note: the "yellow" seen may be either misc_init_r
+(y=300) or the post-clear cyan band (0x00ffff00 = yellow in xRGB,
+y=260) — irrelevant since no clear happened.
+
+### Round 24 payload (`03fc7b4d…`, flashed, cmp+vbutil OK)
+
+New checkpoints (mt8183_scanout.c + video-uclass.c video_post_bind):
+- **purple** (y=470): a video device got BOUND (driver matched DT)
+- **magenta** (y=500): driver probe entered
+- **gray** (y=620): geometry resolved (coreboot LBIO parsed + scanout
+  addr valid)
+- **red bands** at failure exits: y=780 (ovl reg read failed), y=800
+  (coreboot LBIO not found), y=820 (pixel format mismatch), y=840 (no
+  plausible scanout address)
+
+### Decision tree
+
+| observation | meaning |
+|---|---|
+| no purple | video driver never bound (DT compatible / DM scan issue) |
+| purple, no magenta | bound but probe not called (uclass sequencing) |
+| magenta + red@800 | coreboot table/LBIO not found (map/parse) |
+| magenta + red@820 | LBIO found but format mismatch |
+| magenta + red@840 | no plausible scanout address |
+| magenta + gray | geometry OK; failure later (clear/probe tail) |
+| cyan/white appear | video_clear + post_probe reached — back on the R19/20 tree |