The 68000 decoder draws pixel-exact frames, and does not fit

src/player/decode.s parses DLX1 and decodes straight into GVRAM. Verified
pixel-exact over a 120-frame sequential run of the worst sustained window on
the disc -- all four block modes, full temporal recursion, so the last frame
is only right if all 120 were. In check.sh.

It costs a mean of 81.7% of a 12fps frame budget, and 31% of frames exceed
100% (42% at scsi). CPU is now the binding constraint. FINDINGS 28.

Three things that were believed and are not true:

- The dual-display-path plan of FINDINGS 24.5/25.6 is incoherent. The compose
  path needs a RAM copy of the previous reconstruction; the direct path's
  selling point is that it keeps none. Mixing them shows stale pixels on 70 of
  120 frames, worst frame 18.8% of the screen. Every coherent repair is dearer
  than not mixing, and 24.5's two figures were both copies with no decode in
  either, so there was never a crossover to find. One path ships, and the 96KB
  reference frame is gone. tools/analysis/10_pathmix_drift.py keeps the
  counterexample runnable; check.sh asserts it still reproduces.

- The four block modes do not cost the same. V1 300, V4 448, RAW 400 cycles
  against the old model's flat 207.8. V4 is 25% of blocks and 50% of the
  cycles, and the mode decision charges it bytes it does not charge cycles for.
  tools/analysis/11_cpu_budget.py reproduces all four frames timed on the
  68000 to within 1 point. Hand-derived timings agree to 0.5% on V1.

- The container is big-endian but not aligned. Variable-length records laid end
  to end put frame 1's length field at an odd address, and move.l (a0)+ there
  is an address error: frame 0 decoded perfectly and then vectored into the
  IPL for 59 emulated seconds looking like a hang. Found by dumping PC, not by
  reading the source.

Also: an all-V1 frame, the cheapest possible full redraw, is 110.5% of budget.
No mode assignment fits a scene cut at 12fps. That one needs a decision, not a
measurement.

Next: charge cycles in the mode decision and bisect against 833,333 per frame,
the way session 6 bisects lam against bytes -- but with no bucket, because a
late frame cannot be banked.

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

tools/bench/check.sh

@@ -40,4 +40,34 @@ python3 tools/analysis/09_ratectl_drift.py > tmp/drift_check.log 2>&1 \
   || { cat tmp/drift_check.log; exit 1; }
 tail -9 tmp/drift_check.log
 
+echo "--- session 7: display-path coherency (FINDINGS 28.1) ---"
+# 10_pathmix_drift.py is a COUNTEREXAMPLE, kept runnable: the dual-path plan of
+# FINDINGS 24.5/25.6 must still be shown to corrupt frames, and the strategy the
+# player actually uses must still be clean. A green light here means the reason
+# decode.s has one display path is still demonstrable, not just asserted.
+python3 tools/analysis/10_pathmix_drift.py > tmp/pathmix.log 2>&1 \
+  && { echo "FAIL: the dual-path plan no longer reproduces its own defect"; \
+       cat tmp/pathmix.log; exit 1; }
+grep -a "frames displaying pixels" tmp/pathmix.log
+python3 tools/analysis/10_pathmix_drift.py --fix direct > tmp/pathmix_direct.log 2>&1 \
+  || { echo "FAIL: direct-to-GVRAM is no longer coherent"; cat tmp/pathmix_direct.log; exit 1; }
+
+echo "--- session 7: 68000 decoder is pixel-exact (FINDINGS 28) ---"
+# The strongest display test in the tree: 120 frames decoded in sequence by
+# 68000 code, every block mode, full temporal recursion. A SKIP block is a claim
+# about the previous frame still being on screen, so the last frame is only
+# right if all 120 were.
+DLX=tmp/rc_fr_singe_sasi_rcprofile.dlx
+[ -f "$DLX" ] || python3 tools/encoder/encode.py tmp/fr_singe "$DLX" --profile sasi
+python3 tools/bench/prep_dlx.py "$DLX" > tmp/prep_dlx.log
+tools/vasm/vasmm68k_mot -Fbin -o tmp/decode.bin src/player/decode.s > /dev/null
+mkdir -p tmp/snap_decode
+rm -f tmp/snap_decode/x68000/*.png
+( cd tmp && DLX_VERIFY_ONLY=1 SDL_VIDEODRIVER=dummy timeout -k 5 300 mame x68000 \
+    -bios ipl10 -ramsize 2M -video soft -window -sound none -nothrottle -plugins \
+    -autoboot_script ../tools/bench/decode.lua \
+    -snapshot_directory ./snap_decode -snapview native -seconds_to_run 20 \
+    > decode_check.log 2>&1 )
+python3 tools/bench/verify_decode.py "$DLX"
+
 echo "ALL GREEN"