re(media): the disc ships movies in TWO audio profiles, and 28 of them are 5.1

Probed all 97 movies. 28 are wmapro 48 kHz 6-channel 5.1 -- ADV.wmv and every
S*.wmv story cutscene; the other 69 are wmav2 48 kHz stereo, every RT*.wmv and
hokyu_*.wmv. The split is cinematics vs in-mission radio chatter.

Both movies the menu milestone needs, ADV.wmv (the boot/attract intro) and
S00A.wmv (the new-game intro), are in the SURROUND group.

Why it matters: one ffmpeg command over dat/movie/ produces two different kinds
of result and records neither. The 69 stereo files pass through unchanged; the 28
surround files get downmixed 5.1 -> stereo by ffmpeg's DEFAULT matrix, folding
centre-channel dialogue into L/R at a weighting nobody chose and which is not
stable across ffmpeg versions. That is a content decision inherited by accident,
so it should be stated explicitly and recorded beside the command.

Credit where due: found by the human while checking a transcode, verified
independently here and widened from one file to the whole disc.

Two METHOD entries from the same episode, both about measurement rather than
format: don't probe a file another process is still writing (a half-written
transcode reported 33 s against a 137 s source, no error, nearly a filed bug),
and a difference-signal RMS is meaningless before cross-correlation alignment
(-34.2 dB against a -25.3 dB source looks like failure and is inconclusive).

docs/re/METHOD.md

@@ -880,3 +880,19 @@ agent's loop prompt, i.e. nowhere durable. See [`README.md`](README.md) for the
   stuck before it was timed. If the change under test is not in the mesh path,
   `build-reborn t -p <crate>` scopes around it — note that `build-reborn test`
   itself passes `--workspace` and will ignore a `-p`.
+
+* **Never measure a file another process is still writing.** `ffprobe` on a
+  half-written transcode reported 33 s against a 137 s source — the shape of
+  catastrophic truncation, with no error and no warning, and it nearly became a
+  filed bug. The tell was the mtime and a climbing packet count, not anything in
+  the tool's output. Before comparing an artifact you did not produce, check that
+  whoever produces it has finished; across two agents sharing a repo this is a
+  race, not an edge case.
+
+* **A difference-signal RMS means nothing until the two are aligned.** Comparing a
+  transcode against its source gave source RMS −25.3 dB and difference RMS
+  −34.2 dB — only ~9 dB down, which looks like a failure. It is inconclusive: a
+  one-sample offset makes the difference nearly as loud as the source. Any such
+  test needs cross-correlation to align first and an agreed downmix, and only then
+  is a pass mark like ">40 dB down" meaningful. Reporting the 9 dB as a result
+  would have been a confident wrong number.