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/roadmap.md

@@ -322,6 +322,14 @@ Phase 1 hands over the extents; this phase works out what each one contains.
       Within the all-zero case, `i16[3] == 0` exactly (5,851) is a third
       distribution at 62%. Three cases: `0x00/0x00`, `0x00/1..255`, and non-zero
       high byte.
+- [x] **The tie packet's length follows from its record**:
+      `vert_ofs + vert_size + scissor_size` quadwords, on **12,927 of 12,927**
+      record pairs whose `strip_count` is zero and **0 of 115** whose it is
+      not -- an exact partition, so the formula is safe when `strip_count == 0`
+      and knows nothing otherwise. This unblocks slicing the payload per
+      packet. It also corrects this file's own retraction of the same formula:
+      the "0 of 13,042" re-measurement had swapped `strip_count` for
+      `scissor_ofs` and blamed the block set.
 - [~] The remaining tie vertex **fields are not named**, and two readings are now
       excluded. `i16[6]`/`i16[7]` are constant across a whole draw packet in
       43%/34% of records, so they are usually per-packet rather than per-vertex.