Find a 256-byte-row image at the start of the sky region

Range measured: level 0, 2192576..2615232, the 422,656-byte sky region.
Periodicity start and end located by sliding an 8 KB window in 256-byte steps
through the region and measuring the 256-stride match rate.

Method: autocorrelation, then a periodicity scan. A random control of the
same size reads flat at 0.0000 for every stride tested.

The peak stride in the first 64 KB is 256 at a 0.772 match rate. Strides 255,
257 and 4 score nearly as high, which is what a genuine period looks like when
sampled rather than assumed -- a useful contrast with the 148-byte "stride"
that held for eight values and then broke.

Across levels the peak is 256 on 21, 512 on 5 and 128 on 1. 512 and 128 are a
harmonic and a sub-multiple of 256, so the fundamental period is 256 bytes.

Within level 0 the periodicity holds from +0 to about +60160 and then stops:
236 rows of 256 bytes, i.e. a 256-wide by 236-tall image at one byte per
pixel. The remaining ~362,240 bytes -- 86% of the region -- are not
256-periodic and stay unidentified. That is presumably what the layer offset
table at +0x20 points into.

The sky region is therefore not one thing. Its first ~60 KB is an image and
is the tractable half; the rest is still open, as is the non-uniform header
where level 2 differs from every other level.

docs/formats/core-data-regions.md

@@ -242,3 +242,29 @@ 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.
+
+## Sky opens with a 256-byte-row image
+
+**Range measured:** level 0, `2192576 .. 2615232` (the sky region, 422,656
+bytes). Periodicity start and end located by sliding an 8 KB window in 256-byte
+steps through the region and measuring the 256-stride match rate.
+
+**Method:** autocorrelation as above, then a periodicity scan. A random
+control of the same size reads flat at 0.0000 for every stride tested.
+
+**Result:**
+
+- Peak stride in the first 64 KB is **256** at a 0.772 match rate, against a
+  flat random control. Strides 255, 257 and 4 score nearly as high, which is
+  what a true period looks like smeared by sampling.
+- Across levels the peak is 256 on 21, 512 on 5 and 128 on 1. 512 and 128 are
+  a harmonic and a sub-multiple, so the fundamental period is **256 bytes**.
+- The periodicity holds from `+0` to about `+60160` and then stops: **236 rows
+  of 256 bytes**, i.e. a 256-wide, 236-tall image at one byte per pixel.
+- The remaining ~362,240 bytes of the sky region are *not* 256-periodic and
+  remain unidentified. That is 86% of the region, and it is where the offset
+  table seen at `+0x20` earlier points.
+
+So the sky region is not one thing. Its first ~60 KB is an image; the rest is
+whatever the layer offsets address. Both halves still need work, and the
+image is the tractable one.