Settle the tie packet length, and correct the retraction that hid it

Range measured: every tie LOD block on the disc -- 2,177 blocks, 13,042
consecutive record pairs.

A record's pointer is where its packet starts, so the distance between
consecutive pointers IS a packet's length, with no reference to any field.
Comparing that against the record's own fields:

  packet_quadwords = vert_ofs + vert_size + scissor_size

  12,927 of 12,927 record pairs whose strip_count is zero
       0 of 115 record pairs whose it is not

The partition is exact: every record with a non-zero strip_count is an
exception and no record with a zero one is. So the fourth term is 0 quadwords
whenever strip_count == 0, and something else -- measured 0..19 quadwords, and
NOT equal to strip_count -- whenever it is not. R&C1's strip_count is a strip
count and strips are variable-length data, so a non-zero count most likely
adds per-strip bytes this cannot see; that is a hypothesis, labelled as one.

TieRecord gains packet_quadwords and packet_quadwords_are_known. The latter is
the honest gate, and the disc test asserts both sides of the partition rather
than the aggregate.

This also corrects this file's own retraction of the same formula, and the
retraction was the error rather than the claim. A formula of this shape was
derived at 61 of 61 pairs, then "retracted" as not reproducing (0 of 13,042).
The re-measurement had swapped strip_count for scissor_ofs in the fourth term
and put the blame on the block set. Re-measured with the term the first pass
had, it matches on 12,927 of 13,042 and the 115 exceptions are exactly the
non-zero strip_count records.

Worth recording because it is the same shape as two other entries in this
file: a retraction is not automatically more careful than the claim it
retracts, and the re-measurement that "disproved" this one changed the thing
being measured. The lesson on the other side is what saved it -- the first
measurement was over 61 pairs, not a population, and had it stayed there the
formula would have looked universal. A small sample and a bad re-measurement,
near-misses in opposite directions.

What this unblocks: the payload can be sliced per packet, which was the
prerequisite for decoding what it holds.

docs/formats/core-data-regions.md

@@ -1280,6 +1280,47 @@ had loaded. Two operational notes worth keeping:
 
 What the records' fields name and what the payload holds are still open.
 
+## The tie packet's length, from its record's fields (2026-10-11)
+
+**Range measured:** every tie LOD block on the disc -- 2,177 blocks, 13,042
+consecutive record pairs.
+
+**Method:** a record's pointer is where its packet starts, so the distance
+between consecutive pointers **is** a packet's length, with no reference to any
+field. Comparing that against the record's own fields gives the length rule.
+
+**Result: `packet_quadwords = vert_ofs + vert_size + scissor_size`, on 12,927
+of 12,927 record pairs whose `strip_count` is zero -- and on 0 of 115 whose it
+is not.** The partition is exact: every record with a non-zero `strip_count` is
+an exception and no record with a zero one is. The fourth term is 0 quadwords
+whenever `strip_count == 0`, and something else (measured 0..19 quadwords, and
+*not* equal to `strip_count`) whenever it is not. R&C1's `strip_count` is a
+strip count and strips are variable-length data, so a non-zero count most
+likely adds per-strip bytes this cannot see -- a hypothesis, labelled as one.
+
+`TieRecord::packet_quadwords` and `TieRecord::packet_quadwords_are_known` carry
+it; the latter is the honest gate, and the disc test asserts both sides of the
+partition.
+
+**This settles a dispute inside this file, and the retraction was the error.**
+A formula of this shape was derived at 61 of 61 pairs, then "retracted" as not
+reproducing -- re-measured at 0 of 13,042. The retraction was wrong: the
+re-measurement had swapped `strip_count` for `scissor_ofs` in the fourth term,
+and the blame went on the block set. Re-measured with the term the first pass
+had, it matches on 12,927 of 13,042, and the 115 exceptions are precisely the
+non-zero `strip_count` records.
+
+The lesson is worth keeping because it is the same shape as two others here: a
+retraction is not automatically more careful than the claim it retracts, and the
+re-measurement that "disproved" this one changed the thing it was measuring.
+The lesson on the other side is the one that saved it: the first measurement
+was over 61 pairs, which is not a population -- and had it stayed at 61 the
+formula would have looked universal. Both the small sample and the bad
+re-measurement were near-misses in opposite directions.
+
+What this unblocks: the payload can now be sliced per packet, which was the
+prerequisite for decoding what it holds.
+
 ### What the payload is, and what retired the coordinate reading
 
 The payload is **not random**, and that took a control to establish. Reading each