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/encoder/extract.py

@@ -4,9 +4,14 @@
 Source is 1920x1080 (16:9). The arcade original is 4:3, so we CENTER-CROP to
 1440x1080 by default -- see docs/STATUS.md open question on framing.
 """
-import subprocess, sys, os, shutil
+import subprocess, sys, os, shutil, getpass
 
-STREAM_DIR = "/media/reala-misaki/BDROM/BDMV/STREAM"
+# Where the Dragon's Lair Blu-ray is mounted. Nothing in this repo ships the
+# media -- bring your own disc, loop-mount it read-only, and point DLX_BDROM at
+# the mount if it is not where udisks puts it:
+#   udisksctl loop-setup -r -f DRAGONS_LAIR.iso
+BDROM = os.environ.get("DLX_BDROM") or f"/media/{getpass.getuser()}/BDROM"
+STREAM_DIR = f"{BDROM}/BDMV/STREAM"
 W, H = 256, 192
 
 def duration(path):