re: a music bank is two stems that play together -- not intro+loop, and

not something to concatenate

Q10, and it starts by withdrawing the question's own premise. BGM_001 is
not three sub-waves of 10 KB / 4.47 MB / 4.67 MB: the 10 KB is the bank
header. A bank is exactly TWO waves, and across all 32 BGM banks on the
disc the two always have the SAME duration -- equal to 0.01 s over
lengths from 37 s to 277 s. That alone kills intro+loop and kills two
halves of one piece, both of which require unequal lengths.

Four banks appear to break the rule and do not: BGM_106-109 are the known
leading-region straddle, and the giveaway is that the entry named
BGM_107.slb contains BANK id=1108. The seek packet counts pin each join
exactly, so they realign to the same two-equal-waves shape rather than
being dropped as noise. That trap goes in METHOD.

Then the roles, by decoding both waves to PCM. They are
sample-synchronous: transient-envelope correlation searched over +/-5 s
peaks at lag +0.00 s, and both waves stop at the same millisecond,
167.663 s. Two stems of one performance, meant to sound at once. Wave 1
is quieter, has almost no bass and is far more L/R-decorrelated, which
reads as a surround-rear pair or a second intensity layer -- I cannot
separate those two from the file, and say so: ChannelMask is 0x0002 on
both, and this game's channel metadata is already documented as
meaningless.

Two things the port needs that are NOT on the disc, both marked as
authored: the track is not a seamless loop (BGM_001 fades out and is
followed by 6.15 s of silence, no loop-point field found), and nothing
names which bank the menu plays -- all 32 BGM cues are numeric.

docs/port/HANDOFF.md

@@ -36,7 +36,7 @@ authored version can be deleted.
 | Q7 | transitions | ✅ answered | a **fade through black**, drawn by the screen's own last-painting `.prm` quad. Fade-in ramp is **decoded** from its keyframes; the ~0.4 s fade-out is **measured** (not in the file) — [`screen-transitions.md`](../re/screen-transitions.md) |
 | Q8 | menu audio bindings | ❔ open | cue table complete, event binding is not |
 | Q9 | video binding + playback rules | 🟡 partial | `ADV.wmv` is the boot intro; new-game intro unidentified |
-| Q10 | music-bank sub-wave roles (intro+loop?) | ❔ open | we concatenate blindly today |
+| Q10 | music-bank sub-wave roles (intro+loop?) | ✅ answered | **two stems of one performance, played together** — sample-synchronous, equal duration, 32/32 banks. **Concatenating is wrong.** Not a seamless loop either — [`structures/bgm-two-stems.md`](../re/structures/bgm-two-stems.md) |
 | S1 | Ready Room go/no-go | ❔ open | probe not run |
 
 ## Already settled — the port can rely on these today
@@ -140,11 +140,22 @@ authored version can be deleted.
   reader must allow a short read there and only there.
 * **Voice downmixes to mono, music does not.** The left-channel downmix is correct
   for spoken lines and discards half a music mix.
-* **A music bank has several sub-waves and we glue them together.** `BGM_001`
-  is 10 KB + 4.47 MB + 4.67 MB, concatenated into one 347 s track. Nobody has
-  established whether those are intro + loop, two variations, or two halves —
-  see Q10. Do not build menu looping on the concatenated track until it is
-  answered.
+* **A music bank is TWO STEMS THAT PLAY TOGETHER — do not concatenate.**
+  ✅ The "10 KB + 4.47 MB + 4.67 MB" reading was wrong: the 10 KB is the bank
+  header, and a bank is exactly **two waves of identical duration** (32/32 banks
+  on the disc). `BGM_001`'s two are **sample-synchronous** — transient
+  correlation peaks at lag 0.00 s over ±5 s, and both stop at the same
+  millisecond, 167.663 s. Wave 1 is quieter, far more L/R-decorrelated and has
+  almost no bass, so it reads as a surround-rear pair or a second intensity
+  layer — 🟡 which of those is unsettled, and `ChannelMask` is `0x0002` on both,
+  so the file will not say. Today's 347 s concatenation plays the piece twice,
+  the second time as a bass-less stem.
+  ❔ **And it is not a seamless loop**: `BGM_001` fades out at 167.663 s and is
+  followed by 6.15 s of silence, with no loop-point field identified. A menu loop
+  is authored.
+  ❔ **Which bank is the menu's music is not on the disc** — all 32 BGM cues are
+  named `BGM_001`…`BGM_109`, no semantic name in `SOUNDS`, `FILES` or the bank
+  headers. The port is choosing a track.
 * **`JNGL_001.slb` does not decode.** One bank in 9 519; its payload is not a whole
   number of XMA1 packets from any known data offset.