port: pin the 5.1 downmix matrix explicitly -- human decision

The RE agent escalated this rather than picking, correctly: folding centre
into L/R changes how dialogue sits against the music, which is an aesthetic
judgement about the game and not a container detail.

Decided: explicit ITU fold, centre at -3 dB, recorded in the manifest with the
rest of the transcode command. Pinned rather than defaulted because a default is
a decision nobody made -- invisible in the output and free to change between
ffmpeg versions.

docs/MISSION.md

@@ -121,6 +121,28 @@ your own authority.
 Only the boot intro and the one new-game intro are in scope. The disc holds
 3.3 GB of video; transcoding all of it is not this milestone.
 
+### The downmix is decided: pin it explicitly
+
+**Human decision, 2026-08-29.** The cinematics are 5.1 (see
+[`movie-audio-channels`][mac] for the disc-wide split — 28 surround, 69 stereo,
+and *both* movies this milestone needs are surround). Fold to stereo with an
+**explicit matrix**, not ffmpeg's default:
+
+```
+-af "pan=stereo|FL=0.707*FC+1.0*FL+0.707*FLC+0.707*BL+0.707*SL|FR=0.707*FC+1.0*FR+0.707*FRC+0.707*BR+0.707*SR"
+```
+
+Centre at −3 dB into both channels, which is the standard ITU fold and keeps
+dialogue sitting correctly against the music. Record the full command in the
+manifest, per the rule above.
+
+Pinned rather than left to the default because a default is a decision nobody
+made: it is invisible in the output, it can change between ffmpeg versions, and
+it silently alters how speech sits in the mix. Adjust the matrix if it sounds
+wrong — but adjust it *deliberately*, as a commit.
+
+[mac]: https://git.mc02.dev/fabi/Syplheed-Reborn/src/branch/main/docs/re/structures/movie-audio-channels.md
+
 ## 7. Out of scope
 
 3D, gameplay, HUD, missions, save/load, localisation beyond English, the Ready