Characterise sky sub-object 0: sparse, with duplicated entries

Range measured: level 0, sky region +365760..+388880, 23120 bytes, read as
2890 records of 8. The matched-control autocorrelation puts the peak at
stride 8 (0.217 real, 0.000 shuffled), so 8 is the unit to describe.

Composition: 1185 distinct u32 values across 5780 words; a mean of 6.99 of
every 8 bytes are zero, so most records are sparse; and 549 of 2890 adjacent
pairs, 19%, are exact duplicates of each other. The most common 8-byte record
appears 283 times and is d7d7d780 d7d7d780 -- a 4-byte value stored twice.
Others follow the same shape: 00051980 00051980, 00082580 00082580.

Not decoded. Sparsity with repeated entries is what a keyframe or index table
looks like, but nothing here establishes that, and the duplicated-4-byte shape
may just as easily be two adjacent records sharing a value.

Recorded as a characterisation with its range rather than a finding, because
that is what it is. The useful part is that the next attempt does not have to
re-derive the composition to know whether a decode is plausible.

docs/formats/core-data-regions.md

@@ -355,3 +355,28 @@ content rather than being fixed.
 **Level 2's header remains genuinely different**: its count field at `+0x0c`
 reads 196,617 and is unusable, while every other level yields a plausible
 count. Whatever makes it different is still unexplained.
+
+### Sky sub-object 0: sparse, with duplicated entries
+
+**Range measured:** level 0, sky region `+365760 .. +388880`, 23,120 bytes,
+read as 2,890 records of 8.
+
+**Method:** record composition, plus the adjacent-pair and zero-density
+statistics below. The matched-control autocorrelation puts the peak at stride
+8 (0.217 real, 0.000 shuffled), so 8 is the unit to describe.
+
+**What it is made of:**
+
+- **1,185 distinct `u32` values** across 5,780 words.
+- **Mean 6.99 of 8 bytes are zero**, so most records are sparse -- one or two
+  live bytes in eight.
+- **549 of 2,890 adjacent pairs (19%) are exact duplicates of each other.** The
+  most common 8-byte record, appearing 283 times, is `d7d7d780 d7d7d780` -- a
+  4-byte value stored twice. Others follow the same shape: `00051980 00051980`,
+  `00082580 00082580`.
+
+**Not decoded.** Sparsity plus repeated entries is what a keyframe or index
+table looks like, but nothing here establishes that, and the duplicated-4-byte
+shape may equally be two adjacent records that happen to share a value.
+Recorded as a characterisation, with its range, so the next attempt starts
+from the composition rather than re-deriving it.