handoff: the stem question is narrowed, not settled -- and it does not change what the port does

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Wuu56cE8vJGTBtn1ppsk8v

docs/port/HANDOFF.md

@@ -746,6 +746,32 @@ a bind mount on another device, and which is what the withdrawn banner ran.
 `sylpheed-cli` against a pak, run the disc-gated tests and read the executable
 again. New measurements are available; ask for them.
 
+## 🟡 2026-08-30 — the second stem is NOT a filtered copy, and Q10's row was stale
+
+**Nothing here changes what you do with the two waves.** Both surviving readings
+give the same instruction, unchanged: **play both, aligned at sample 0, together.**
+This only bears on how they would be *mixed* if you ever do surround.
+
+✅ **MISSION's Q10 row was stale and I have corrected it.** It still described a
+bank as *three* sub-waves and asked whether they are intro + loop, two variations,
+or two halves — every one of those refuted, some for days. Its gate is in fact met.
+Read the row's replacement, not the row.
+
+❌ **"Wave 1 is wave 0 through a filter" is refuted** on `BGM_103`, your menu bank.
+Coherence, controlled first: a real linear filter of wave 0 reads **0.93–0.94** in
+every band, a different bank **0.001**, wave 0 misaligned by 1 s **0.004–0.057**.
+The measurement reads **0.027** at 1–4 kHz. Coherence also *rises* with frequency
+(0.169 → 0.827) while energy *falls* (71 % → 0.2 %) — inverted relative to any
+mic-pair or reverb model.
+
+🔴 **And I could not separate the two readings — the limit is in the instrument,
+not in the data.** L vs R *within* one wave is one performance in two channels and
+reads only **0.221–0.497**, so "same performance" does not imply high coherence in
+this material; my positive control was the wrong model of a rear pair. If you see
+this test cited later as having settled the stem question, it did not.
+[`data/bgm-stem-coherence.txt`](../re/data/bgm-stem-coherence.txt) ·
+[`structures/bgm-two-stems.md`](../re/structures/bgm-two-stems.md)
+
 ## ✅ 2026-08-29 — the "third sub-wave" on a music bank was OUR reader, and it is fixed
 
 **You were right to refuse to choose which one to drop.** `BGM_103.slb` really