Take the disc mount out of the source, so someone else's copy of the media works

Three files hardcoded /media/reala-misaki/BDROM -- extract.py, 07_motion_survey.py
and check.sh -- which made the pipeline unrunnable for anyone whose disc mounts
anywhere else. DLX_BDROM now overrides it everywhere, defaulting to
/media/$USER/BDROM, so this box is unchanged and another one works. Verified by
running extract.py against a symlinked mount at a different path.

check.sh's failure message names the path it looked at and the variable to set,
instead of assuming udisks put it where this machine puts it.

README gains a "Reproducing this" section: no media ships here and none of it is
redistributable, so it says what you have to bring (the disc) and what is
already packaged (vasm is vendored as a binary with its source tarball; the
k-means is hand-rolled, so numpy and Pillow are the whole Python dependency).
It also names the two gates that SKIP rather than fail -- the px68k second-core
pass and the IPL ROM DMAC gate -- because both live outside this repo and a
silent skip is worth reading as a skip.

One trap called out rather than left to bite: scene selection is a hard-coded
stream number, not a search. A different pressing that numbers its .m2ts files
differently will extract the wrong footage and the green light will PASS on it.

check.sh ALL GREEN.

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

tools/bench/check.sh

@@ -4,8 +4,14 @@
 # exit means something drifted.
 set -e
 cd "$(dirname "$0")/../.."
-[ -d /media/reala-misaki/BDROM ] || {
-  echo "Blu-ray not mounted. udisksctl loop-setup -r -f DRAGONS_LAIR.iso"; exit 2; }
+# No media ships with this repo. Bring your own disc; DLX_BDROM overrides the
+# mount point, and every tool that reads the disc honours the same variable.
+DLX_BDROM=${DLX_BDROM:-/media/${USER:-$(id -un)}/BDROM}
+export DLX_BDROM
+[ -d "$DLX_BDROM" ] || {
+  echo "Blu-ray not mounted at $DLX_BDROM."
+  echo "  udisksctl loop-setup -r -f DRAGONS_LAIR.iso"
+  echo "  or set DLX_BDROM to where yours is mounted."; exit 2; }
 
 python3 tools/encoder/extract.py 00020 tmp/fr_00020 12 crop
 mkdir -p tmp/snap_verify tmp/snap256