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/analysis/07_motion_survey.py

@@ -13,10 +13,12 @@ window it picks then gets encoded for real.
 
 Usage: python3 tools/analysis/07_motion_survey.py 00223 [window_seconds]
 """
-import subprocess, sys
+import subprocess, sys, os, getpass
 import numpy as np
 
-STREAM_DIR = "/media/reala-misaki/BDROM/BDMV/STREAM"
+# See tools/encoder/extract.py: DLX_BDROM overrides where the disc is mounted.
+BDROM = os.environ.get("DLX_BDROM") or f"/media/{getpass.getuser()}/BDROM"
+STREAM_DIR = f"{BDROM}/BDMV/STREAM"
 W, H, FPS = 96, 72, 12
 
 def frames(stream):