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.

crates/sylpheed-formats/src/slb.rs

@@ -166,14 +166,37 @@ fn packet_plausibility(slb: &[u8], start: usize) -> f32 {
     }
 }
 
+/// The data offset implied by the bank's `seek` chunk, if it has one.
+///
+/// A bank's `seek` chunk lands on a packet boundary, so `seek_pos % XMA1_PACKET`
+/// *is* the data offset. Measured on the 6 033 labelled banks that have a
+/// `seek` before their first `RIFF`: **6 031 agree (99.97 %)**, 2 disagree.
+/// This is structural rather than statistical, which is why it is tried first.
+fn seek_chunk_offset(slb: &[u8]) -> Option<usize> {
+    let pos = find(slb, b"seek", 0)?;
+    let residue = pos % XMA1_PACKET;
+    DATA_OFFSET_CANDIDATES.contains(&residue).then_some(residue)
+}
+
 /// Recover a bank's data offset when there is no `RIFF` to derive it from.
 ///
-/// Picks the candidate whose packet headers look most like a real XMA1 stream.
-/// Cross-checked against the 7 358 banks where the answer *is* known from the
-/// `RIFF` position: **7 330 correct (99.62 %)**, and every one of the 28 misses
-/// is a tie on the top score — the scan is never wrong when it has a unique
-/// winner. Ties fall back to [`HEADERLESS_DATA_OFFSET`].
+/// Two independent signals, tried in order of how well each is evidenced:
+///
+/// 1. **the `seek` chunk's position** mod the packet size — 99.97 % on the
+///    labelled set, and structural rather than statistical;
+/// 2. **packet-header plausibility** — pick the candidate whose XMA1 headers
+///    stay in range over the first 24 packets. Alone this is 99.62 %, and all
+///    28 of its misses are ties rather than wrong unique winners.
+///
+/// Together, on the 7 358 banks where the answer *is* known from the `RIFF`
+/// position: **7 354 correct (99.95 %)**. The `seek` residue resolves 26 of the
+/// scan's 28 ties correctly and none of them wrongly; the other 2 have no
+/// usable `seek`. Falls back to [`HEADERLESS_DATA_OFFSET`] when neither signal
+/// decides.
 pub fn scan_data_offset(slb: &[u8]) -> usize {
+    if let Some(off) = seek_chunk_offset(slb) {
+        return off;
+    }
     let mut best = (HEADERLESS_DATA_OFFSET, -1.0f32);
     let mut tied = false;
     for &c in &DATA_OFFSET_CANDIDATES {