Handoff: green-light script, reconciled figures, a smaller first step for the decoder

Prepares session 4 for handoff. No new measurements; this reconciles the docs
with what session 4 changed and makes the next session's entry point cheaper.

- tools/bench/check.sh re-runs both display regression tests from the Blu-ray in
  ~40 s and prints ALL GREEN. Verified green cold, after wiping tmp/ and
  re-extracting. STATUS and README both open with it, because everything
  downstream assumes the display path is pixel-exact and nothing previously
  checked that in one command.

- Reconciled the figures session 4 invalidated. Session 3's 38.88 dB ceiling is
  struck through in STATUS with a pointer to 40.81; the "three facts the player
  must honour" table no longer quotes R20 = 0x0116, which was the 768-wide IPL
  timing and would have been copied into the player as if it were the shipping
  value. crtc_mode.lua is now named as the single source of truth for CRTC
  registers, in both STATUS and README.

  The 38.88 dB in the session-3 reproduce section is left alone and annotated
  instead: it is correct for that test, which still packs I = 1. The two numbers
  disagree for a reason and a reader should be able to see which is which.

- Next-step 2 now leads with something smaller than "write the decoder": a dumb
  full-frame RAM->GVRAM blit in 68000 code, timed. That number alone confirms or
  kills the 38% estimate, and needs no bitstream, codebooks, or DLX1 parsing.
  The reference image and its checker already exist.

- Parked the user's Cliff Hanger / Lupin III follow-on in STATUS so it is not
  lost and not mistaken for scheduled work. Cheaper than this project on every
  axis except media prep, which is where it would actually stall.

Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6

tools/bench/check.sh

@@ -0,0 +1,30 @@
+#!/bin/bash
+# Green-light check: re-runs both display regression tests from the Blu-ray.
+# ~40 s. Run from the repo root. Any non-zero exit means something drifted.
+set -e
+cd "$(dirname "$0")/../.."
+[ -d /media/reala-misaki/BDROM ] || {
+  echo "Blu-ray not mounted. udisksctl loop-setup -r -f DRAGONS_LAIR.iso"; exit 2; }
+
+python3 tools/encoder/extract.py 00020 tmp/fr_00020 12 crop
+mkdir -p tmp/snap_verify tmp/snap256
+
+run() {  # run <script> <snapdir>
+  rm -f "tmp/$2/x68000"/*.png
+  ( cd tmp && SDL_VIDEODRIVER=dummy timeout -k 5 120 mame x68000 -bios ipl10 \
+      -video soft -window -sound none -nothrottle -plugins \
+      -autoboot_script "../tools/bench/$1" \
+      -snapshot_directory "./$2" -snapview native -seconds_to_run 6 >"$2.log" 2>&1 )
+}
+
+echo "--- session 3: 768-wide IPL timing (FINDINGS 22) ---"
+python3 tools/bench/prep_frame.py tmp/fr_00020 tmp/frame.bin 0
+run show_frame.lua snap_verify
+python3 tools/bench/verify_frame.py
+
+echo "--- session 4: real 256x256 mode (FINDINGS 23) ---"
+python3 tools/bench/prep_frame.py tmp/fr_00020 tmp/frame256.bin 0 --reserve-black
+run show_frame256.lua snap256
+python3 tools/bench/verify_frame256.py
+
+echo "ALL GREEN"