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/re/structures/bgm-two-stems.md

@@ -0,0 +1,115 @@
+# A music bank is two stems of one performance, played together
+
+**Status:** ✅ `CONFIRMED` for the structure — **decoded**, with a disc-wide check
+over all 32 BGM banks. 🟡 for the *role* of the second wave — **measured** by
+signal analysis, and the two readings that survive are named below. ❔ which bank
+is the menu's music is **undecodable from the disc**, with the reach stated.
+
+Answers [MISSION Q10](../port/MISSION.md). It also **withdraws the premise**: the
+handoff said `BGM_001.slb` is *three* sub-waves of 10 KB / 4.47 MB / 4.67 MB. The
+10 KB is the bank **header**, not a wave.
+
+## The structure — decoded
+
+```
+BGM_001.slb   (9 178 040 B)
+      0   BANK  id=1001  data_size=9175992  hdr = 5 blocks (10240 B)  16bit/2ch
+  10240   RIFF  data@14336    size=4466688  (2181 packets)   25697 B/s → 173.82 s
+4481024   seek  2181 packets
+4491264   RIFF  data@4495360  size=4673536  (2282 packets)   26887 B/s → 173.82 s
+9168896   seek  2282 packets
+```
+
+A bank is a 10 240-byte header and then **exactly two waves**, and the two always
+have the **same duration** — different byte sizes and different bitrates, same
+number of seconds. Duration is `data_size / PsuedoBytesPerSec` (the u32 at
+`RIFF+0x20`; `RIFF+0x24` is the sample rate, 48 000 Hz except `BGM_020`–`023`
+at 44 100).
+
+### Disc-wide
+
+`tools/re-capture/bgm_wave_census.py`, full output in
+[`../data/bgm-wave-census.txt`](../data/bgm-wave-census.txt):
+
+> **28 of the 32 BGM entries** read straight off their own pak entry as two waves
+> of equal duration — equal to 0.01 s, over lengths from 37 s to 277 s.
+
+The other four — `BGM_106`…`BGM_109` — are **not counterexamples**, they are the
+known leading-region straddle ([`slb-data-offset.md`](slb-data-offset.md)): those
+entry windows start mid-bank, so an entry contains the tail of the previous bank,
+one whole wave of its own, and the `BANK` header of the *next* one.
+`slb_segment_phase.py bank` shows it directly — the entry named `BGM_107.slb`
+contains `BANK id=1108`. Realigning across the boundaries with the `seek` packet
+counts, which pin each join exactly (bank 1107: a `seek` for 1164 packets and a
+2 383 872-byte wave = 1164 packets), restores the same shape:
+
+| bank | wave 0 | wave 1 |
+|---|---|---|
+| 1107 | 2 383 872 B → 65.18 s | 2 418 688 B → 65.18 s |
+| 1108 | 3 696 640 B → 128.91 s | 3 696 640 B → 128.91 s |
+| 1109 | 2 887 680 B → 81.50 s | 2 932 736 B → 81.50 s |
+
+Bank 1106's wave 0 begins in unclaimed bytes before its entry and is known only
+by its `seek` count (1 718 packets), so it is 🟡 rather than measured.
+
+**So: intro + loop is dead, and so is two halves of one piece.** Both would give
+unequal durations; 32 banks give equal ones.
+
+## The role of the second wave — measured
+
+Decoded `BGM_001`'s two waves to PCM and compared them.
+
+| | wave 0 | wave 1 |
+|---|---|---|
+| duration | 173.809 s | 173.809 s |
+| first sound / last sound | 0.001 s / **167.663 s** | 0.003 s / **167.663 s** |
+| RMS | 6 824 | 3 961 |
+| L↔R correlation | 0.61 | **0.14** |
+| energy below 200 Hz | 16.5 % | **6.7 %** |
+| energy 1–4 kHz | 31.1 % | 42.5 % |
+
+**They are sample-synchronous.** Transient-envelope cross-correlation searched
+over ±5 s peaks at **lag +0.00 s** (0.5624), and both waves stop at the *same
+millisecond*, 167.663 s. Two takes, two halves or two alternates would not do
+that. They are two stems of one performance and are meant to sound **at the same
+time**.
+
+**Therefore concatenating them is wrong**, and not subtly: it plays the piece
+twice, the second time as a bass-less secondary stem.
+
+### 🟡 Which kind of second stem — two readings survive
+
+* **The rear pair of a 4-channel mix.** Fits every number: quieter, far more
+  decorrelated between L and R, and with the bass managed away to the front pair.
+* **A second intensity layer** to be mixed in or crossfaded.
+
+**Not settled, and the obvious field does not settle it.** `ChannelMask` is
+`0x0002` on *both* waves, and [`sound-slb.md`](sound-slb.md) already records that
+this game writes meaningless channel metadata (movie voices declare 2 channels
+over mono content). What would settle it is a runtime observation — whether the
+game submits both waves to the mixer at once — which needs an emulator with audio
+this container does not have.
+
+## ❔ Which bank is the menu's music — undecodable, and here is the reach
+
+The cue table binds cue names to sound ids and banks
+([`sound-cue-table.md`](sound-cue-table.md)), and its **32 BGM cues are named
+`BGM_001` … `BGM_109`** — pure numbers, no `TITLE`, no `MENU`, no semantic name
+anywhere. Looked in: the `SOUNDS` record (5 798 cues), `FILES` (5 135 bank
+paths), and the bank headers themselves (a `BANK id` that simply repeats the cue
+number, `id=1001` for `BGM_001`). Nothing names a screen.
+
+So the menu↔bank binding has to come from the running game — the same route Q8's
+event bindings need — and until then the port is **choosing** a track, not
+transcribing one.
+
+## What the port needs to know
+
+* a music bank is **two waves that play together**, not one track and not a
+  sequence — do not concatenate;
+* both are full length; align them at sample 0;
+* **the track is not a seamless loop.** `BGM_001` fades out and is followed by
+  **6.15 s of silence** (last sound 167.663 s of 173.809 s; the final second
+  before silence is at RMS 168 against 4 788 at the head). Looping the wave as
+  stored gives a fade-out and six seconds of nothing every cycle. No loop-point
+  field has been identified in the XMA header, so a menu loop is **authored**.