slb: the seek chunk gives the data offset structurally, and breaks the 28 ties

The ties needed a different signal, not a longer scan. Banks carry one: a seek
chunk sitting on a packet boundary, so seek_pos % 2048 IS the data offset. On
the 6033 labelled banks with a seek before their first RIFF, 6031 agree
(99.97%) -- better than the packet scan and structural rather than statistical,
so scan_data_offset now tries it first.

On the scan's 28 ties it resolves 26 correctly and 0 wrongly (2 have no usable
seek). Combined rule scores 7354/7358 = 99.95%, up from 99.62%. 762 of the 1495
RIFF-less banks carry a seek, so the signal exists where it is needed.

Also ruled out, since a wrong offset was this page's whole subject: the header
is not audio being discarded. Adding 0 to the candidate set, it wins 6 of 7358.

7 disc tests pass.

docs/re/structures/slb-data-offset.md

@@ -120,6 +120,37 @@ with a unique winner. `scan_data_offset` therefore falls back to 1392 on a tie.
 This is used only for the `RIFF`-less banks. Where a `RIFF` exists the offset is
 derived from it exactly, never scanned.
 
+### ✅ A second, independent signal — and it breaks the ties
+
+**Settled 2026-08-26.** The 28 ties needed a different signal, not more of the
+same one, and the banks carry one: a **`seek` chunk sitting on a packet
+boundary**. Its position modulo 2048 therefore *is* the data offset.
+
+    seek at 3 516 / 5 564 / 7 612 / 9 660 / 13 756 / 19 900  —  all ≡ 1468 (mod 2048)
+
+On the 6 033 labelled banks that have a `seek` before their first `RIFF`,
+**6 031 agree (99.97 %)** and 2 disagree. That is better than the packet scan
+and, more importantly, *structural* rather than statistical — which is why it is
+now tried first.
+
+Applied to the packet scan's 28 ties: **26 resolved correctly, 0 wrongly**, and
+2 with no usable `seek`. The combined rule — `seek` residue, else packet
+plausibility, else 1392 — scores **7 354 / 7 358 = 99.95 %** on the labelled set,
+up from 99.62 %.
+
+762 of the 1 495 `RIFF`-less banks carry a `seek`, and its residue lands on the
+four known offsets there too (1468 ×343, 1600 ×255, 1392 ×148, 1728 ×16), so the
+signal is available in the population that needs it.
+
+### ❌ The header is not audio being discarded
+
+Worth ruling out, since a wrong data offset was the whole subject of this page:
+if the bytes *before* the offset were audio, we would be throwing away the start
+of every clip. Adding **0** to the candidate set and re-running the scan, it wins
+**6 of 7 358** — noise. The header is genuinely not part of the packet stream.
+(1 482 banks have an all-zero header; 5 876 have content in it, which is what
+prompted the check.)
+
 ### Is that 99.62 % transferable? — checked, and it is conservative
 
 The labelled set has a `RIFF`; the population the scan actually serves does not.