Show sky sub-object 0 is a slowly-varying sequence at two scales

Range measured: level 0, sky region +365760..+388880, 23120 bytes.

Method: Pearson correlation between the byte sequence and itself at
increasing lag, against a shuffled control. This asks whether consecutive
records are related, which the position-wise distinct-value counts could not
show.

  lag   1  +0.050      lag 256  -0.044
  lag   4  +0.560      lag 512  +0.304
  lag   8  +0.746      lag 1024 +0.247
  lag  16  +0.666      lag 2048 +0.079
  lag  32  +0.602      shuffled +0.003
  lag  64  +0.432
  lag 128  +0.140

The profile peaks at lag 8, decays to a zero crossing near 256, then rises
again to a second smaller peak at 512 before decaying. So records are related
within themselves and repeat every 64 records.

This settles the earlier "no constant substructure" reading. The records share
no fixed fields, but they are not independent either, and that was the thing
the distinct-value counts were failing to capture.

Slowly varying values, related within a record, repeating every 64 records, is
the shape of a keyframe or lookup table -- which is what sky data should be,
where a colour or fog parameter is sampled rather than computed per vertex.

Still not decoded: nothing names the values or the sampling. But the shape is
now known well enough for a decode attempt to aim at, rather than to hunt.

docs/formats/core-data-regions.md

@@ -387,3 +387,45 @@ help: the values span -32743..32633, close to the full range, with no
 per-position structure (533, 371, 401 and 187 distinct at positions 0-3).
 Recorded as a characterisation, with its range, so the next attempt starts
 from the composition rather than re-deriving it.
+
+### Sky sub-object 0 is a slowly-varying sequence, not independent records
+
+**Range measured:** level 0, sky region `+365760 .. +388880`, 23,120 bytes.
+
+**Method:** Pearson correlation between the byte sequence and itself at
+increasing lag, against a shuffled control. This measures whether consecutive
+records are *related*, which the position-wise distinct-value counts could
+not show.
+
+**Result:**
+
+| lag (bytes) | correlation |
+|---|---|
+| 1 | +0.050 |
+| 4 | +0.560 |
+| **8** | **+0.746** |
+| 16 | +0.666 |
+| 32 | +0.602 |
+| 64 | +0.432 |
+| 128 | +0.140 |
+| 256 | −0.044 |
+| **512** | **+0.304** |
+| 1024 | +0.247 |
+| shuffled control (lag 1) | +0.003 |
+
+The profile has a peak at lag 8, decays to a zero crossing near 256, and then
+a second, smaller peak at 512 before decaying again.
+
+This is what settles the earlier "no constant substructure" reading. The
+records do not share fixed fields, but they are *not independent*: consecutive
+8-byte records are strongly related, and that relation repeats at 512 bytes,
+i.e. every 64 records.
+
+Slowly varying values, related within a record, repeating every 64 records, is
+the shape of a keyframe or lookup table -- which is what one would expect to
+find in sky data, where a colour or fog parameter is sampled rather than
+computed per vertex.
+
+**Still not decoded.** Nothing here names the values or the sampling. But the
+structure is now characterised well enough that a decode attempt has a shape
+to aim at rather than a field to hunt for.