Confirm moby's 16-byte vertex by two independent methods

The part table established vertex_offset + 16 * vertex_count == next_part on
43467 of 43467 parts, which fixes the vertex stride at 16. Autocorrelation
over the spans now reaches the same number without using that result.

Range measured: moby class spans of 16 KB or more on level 0 -- 60 spans,
bounded by consecutive class offsets, over their first 32768 bytes.

Method: peak-stride autocorrelation against a matched control, strides 4 to
64.

  stride 16   43 spans
  stride  8    8
  stride 48    4
  stride 32    3
  stride 24    2

16 dominates and it is the same 16 the part table produces from a completely
different argument -- a tiling identity over part records rather than a
periodicity measurement over span contents.

Two unrelated methods agreeing is worth more than either alone. The
autocorrelation would be vulnerable to the exact artifacts this project has
already been bitten by: a dominant byte value inflates it, and a period that
happens to divide the measurement window looks identical to a real one.
Neither applies here, and a tiling identity cannot be inflated at all.

This is the strongest form of evidence produced so far in this project -- a
structural invariant confirmed by two methods that do not share a failure
mode.

docs/formats/core-data-regions.md

@@ -528,3 +528,40 @@ This is weaker evidence than the sound bank's 40-byte result, which was
 sharply isolated and identical on all 27 levels. Here the peak is weaker
 (0.09-0.22 excess against sound bank's 0.45) and it moves between levels and
 spans. Recorded at the confidence it deserves.
+
+## Two independent measurements agree on moby's 16-byte vertex
+
+The moby part table established `vertex_offset + 16 * vertex_count ==
+next_part` on 43,467 of 43,467 parts, which fixes the vertex stride at 16.
+Autocorrelation over the spans reaches the same number without using that
+result at all.
+
+**Range measured:** moby class spans of 16 KB or more on level 0 -- 60 spans,
+each bounded by consecutive class offsets, measured over the first 32,768
+bytes.
+
+**Method:** peak-stride autocorrelation against a matched control, searching
+strides 4 to 64.
+
+**Result:**
+
+| peak stride | spans |
+|---|---|
+| **16** | **43** |
+| 8 | 8 |
+| 48 | 4 |
+| 32 | 3 |
+| 24 | 2 |
+
+16 dominates, and it is the same 16 the part table produces from a completely
+different argument -- a tiling identity over part records rather than a
+periodicity measurement over span contents.
+
+Two unrelated methods agreeing is worth more than either alone, because the
+autocorrelation would be vulnerable to exactly the kinds of artifact this
+project has already been bitten by: a dominant byte value would inflate it,
+and a period that happened to divide the window would look the same as a real
+one. Neither applies here, and the tiling identity cannot be inflated at all.
+
+This is the strongest form of evidence produced in this project: a structural
+invariant confirmed by two methods that do not share a failure mode.