Import the scene graph, and find the worst gap between two branch points is zero

ROADMAP G1, scheduled early because it is a measurement input, and it paid for
that immediately. 51.3 established that a ring's lookahead is accumulated out of
pipe - wire and that a seek spends all of it, so what a branch point costs is
set by the time since the last one. 55.5 rehearsed a seek on the machine and
said out loud that it could not ask the question, because nothing here knew
where the branch points are.

They are everywhere, and 5.4% of them are free of charge to the player and not
to us. Over 612 distinct transitions into a seek, taking the earliest instant
each input window opens: worst 0.000 s, p10 0.950, median 3.473, best 82.497.
33 open on the first frame of a clip the disc SEEKED to -- press right as
flaming_ropes.enter_room appears and you are in fall_to_death, two seeks with no
play between them. 51.2's slack rule can therefore be answered NO by the
content rather than by the buffer, and no amount of ring is a defence.

It does not break the design. A branch on an empty ring costs the 2-record
prefill, 149.7 ms at 488 KB/s, not the climb. What it removes is margin: at that
rate in a 256 KB ring, 76% of this game's branch points arrive before the ring
has refilled, and a 512 KB ring makes that 90%, because doubling the ceiling
does not touch the surplus. The ring is not the lever; the surplus is.

CORRECTION to FINDINGS 16: there is only one transcription. The SNES chapter
set says in its own README that it is derived from DirkSimple, so the planned
diff of two independent sources catches conversion errors only. Run anyway:
durations agree 388/505 within a frame, branch structure 470/505, and of the 35
differences 16 are renames and 18 of the other 19 are that port dropping the
arcade's diagonals. Zero transcription discrepancies, and none were findable.

Two constraints on the input layer come free: the arcade needs eight directions,
and the shortest input window is 98 ms against 54.4's 72.13/90.16 ms frame slot,
so input cannot be polled on the frame tick.

The coupling to outside source is contained to one file (USER DECISION).
tools/import/scenegraph.py is the only code here that knows those projects
exist -- their paths, table names, timing formulas, constants -- and it writes
DLXSCENE1, this project's own schema, into gitignored tmp/ with the sources'
licences inside it. tools/analysis/25_scene_graph.py reads only that. Nothing is
vendored and nothing outside-derived is committed. The split was made after the
measurement and the whole output was re-run byte for byte to show it moved no
number.

Both import gates are negative-tested: deleting one sequence upstream fails the
516/906 count, and closing the table early fails the constructor-end check,
which replaced one that was vacuous.

No 68000 code ran or changed; decode.bin is still 1,296 B at the same MD5.
check.sh gains an import stage that skips when there is no checkout. ALL GREEN
before and after.

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

tools/bench/check.sh

@@ -406,4 +406,30 @@ grep -q "^OK" tmp/ringseek_check.log || {
   echo "FAIL: the decode after the seek was not pixel-exact."
   tail -4 tmp/ringseek_check.log; exit 1; }
 
+echo "--- session 24: the scene graph, and the gap between branch points (FINDINGS 56) ---"
+# The arcade scene graph is not in this repo and is not redistributable from
+# here. tools/import/scenegraph.py is the ONE file in the tree that knows the
+# outside projects exist; it writes tmp/scenegraph.json in this project's own
+# DLXSCENE1 schema and everything downstream reads only that.
+# What is gated is the IMPORT, not the numbers: 516 sequences and 906 input
+# windows, and the four timing helpers still being the formulas the importer
+# evaluates. Skipped when the checkout is absent.
+DIRKSIMPLE=${DLX_DIRKSIMPLE:-tmp/scenegraph/DirkSimple}
+if [ -f "$DIRKSIMPLE/data/games/lair/game.lua" ]; then
+  DLX_DIRKSIMPLE="$DIRKSIMPLE" python3 tools/import/scenegraph.py \
+    -o tmp/scenegraph.json > tmp/scenegraph_import.log 2>&1 \
+    || { cat tmp/scenegraph_import.log; exit 1; }
+  sed "s/^/  /" tmp/scenegraph_import.log
+  grep -q "516 sequences, 906 input windows" tmp/scenegraph_import.log || {
+    echo "FAIL: the scene graph did not import to 516/906 -- upstream changed,"
+    echo "      or the parser silently dropped branches."; exit 1; }
+  python3 tools/analysis/25_scene_graph.py --kbps 488 --ring 256 \
+    > tmp/scenegraph_check.log 2>&1 \
+    || { tail -20 tmp/scenegraph_check.log; exit 1; }
+  grep -aE "^  WORST |^  ZERO-PLAY|^  BRANCH STRUCTURE" tmp/scenegraph_check.log
+else
+  echo "  SKIPPED: no DirkSimple checkout at $DIRKSIMPLE"
+  echo "           (git clone --depth 1 https://github.com/icculus/DirkSimple)"
+fi
+
 echo "ALL GREEN"