Differential analysis finds no constant substructure at stride 368

Range measured: level 0, 16047040..16737344. 1875 whole 368-byte records
from the region start, variation checked across the first 60.

Method: the differential analysis that identified the shrub body -- compare
records byte by byte, record which positions differ.

Result: all 368 positions vary. No constant field, no padding, no header
inside a record. The shrub body separated immediately into a varying payload
and a constant two-byte tail; this format has nothing constant to separate.

That is consistent with the autocorrelation peak rather than in tension with
it. Checking which positions carry the 368-stride matches over 20000
positions gives 168 exact 4-byte repeats, distributed evenly at 0.84% across
all four byte offsets. Whole 4-byte sequences recur at exactly 368 while no
individual position is constant -- consistent with records containing entries
that repeat at a fixed relative index, not with a fixed-layout record.

So the 368 stride stands and the records stay unidentified. Worth recording
that the shrub method failed here for a specific and predictable reason: it
needs something constant to latch onto. Applying a method that worked
elsewhere is not the same as it applying here, and saying so is cheaper than
the next person assuming the technique transfers.

docs/formats/core-data-regions.md

@@ -218,3 +218,27 @@ 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.
+
+### Differential analysis at stride 368: no constant substructure
+
+**Range measured:** level 0, `16047040 .. 16737344`. 1,875 whole 368-byte
+records from the region start; variation checked across the first 60.
+
+**Method:** the same differential analysis that identified the shrub body --
+compare records byte by byte and record which positions differ.
+
+**Result:** **all 368 positions vary.** There is no constant field, no padding
+and no header inside a record. Unlike the shrub body, where differential
+analysis immediately separated a varying payload from a constant two-byte
+tail, this format has nothing constant to separate.
+
+That does not contradict the autocorrelation peak. Checking which byte
+positions carry the 368-stride matches over 20,000 positions: 168 exact 4-byte
+repeats, distributed evenly at 0.84% across all four byte offsets. So whole
+4-byte sequences recur at exactly 368 while no individual position is constant
+-- consistent with records containing entries that repeat at a fixed relative
+index, rather than with a fixed-layout record.
+
+**So the 368 stride stands and the records remain unidentified.** The shrub
+method failed here not because it was the wrong tool but because it needs
+something constant to latch onto, and there is nothing.