Find a repeating structure at stride 368 in the region after shrub

Range measured: level 0, 16047040..16737344, sampled as the first 65536
bytes. Cross-checked on every level that has the region, each sampled the
same way from its own region start.

Method: autocorrelation. For each candidate stride, count how often a 4-byte
sequence at offset i repeats at i + stride, as a fraction of comparable
positions. A random control of the same size measured identically reads flat
at 0.0000 for every stride, so the statistic does discriminate.

  stride 368        0.1110
  stride 340        0.0935
  strides 337-339   0.026-0.029   background
  random control    0.0000

Stride 368 is the strongest peak. On level 0, 340 is a secondary peak well
clear of the background; the two differ by 28, unexplained.

The cross-check is the part that matters: the peak stride was recomputed
independently for every level with the region, and all 24 peak at exactly
368. One level's peak could be a property of that level's data; 24 of 24
agreeing is a format invariant.

What a 368-byte record is remains unestablished. This is the same position
the shrub body was in before differential analysis identified its records,
and the same method is the obvious next step -- apply it to the 368 stride
rather than continuing to look for headers.

Chosen deliberately over the header search: autocorrelation found this in one
pass with a control, where four header guesses found nothing. The shrub body
only yielded to differential analysis too, and pattern-matching is what
produced this session's four retractions.

docs/formats/core-data-regions.md

@@ -185,3 +185,36 @@ less than a structure needs.
 Recorded because the lead was committed as a lead and this is where it goes.
 Nothing else about the region has been identified: it is real, non-zero data
 of unknown type, bounded, and unopened beyond these ~150 bytes.
+
+## The region after shrub has a repeating structure at stride 368
+
+**Range measured:** level 0, `16047040 .. 16737344`, sampled as the first
+65,536 bytes. Cross-checked on every level that has the region, each sampled
+the same way from its own region start.
+
+**Method:** autocorrelation. For each candidate stride, count how often a
+4-byte sequence at offset *i* repeats at *i + stride*, as a fraction of
+comparable positions. A random control of the same size is measured
+identically and reads flat at 0.0000 for every stride, so the statistic
+discriminates.
+
+**Result:**
+
+| stride | region match rate |
+|---|---|
+| 368 | 0.1110 |
+| 340 | 0.0935 |
+| 337–339 | 0.026–0.029 (background) |
+| random control | 0.0000 |
+
+Stride **368** is the strongest peak. On level 0, 340 is a secondary peak
+well clear of the background; the two differ by 28, which is unexplained.
+
+**Cross-check:** the peak stride was recomputed independently for every level
+that has the region, and **all 24 peak at exactly 368**. That is the part
+that makes this worth recording -- a single level's peak could be a property
+of that level's data, but 24 of 24 agreeing is a format invariant.
+
+What a 368-byte record *is* remains unestablished. This is the same position
+the shrub body was in before differential analysis identified its records,
+and the same method is the obvious next step.