Correct the sky sub-object 0 live-byte figure; record the byte 7 anomaly

The previous commit said "a mean of 6.99 of every 8 bytes are zero". That
is inverted. 6.99 is the mean count of live bytes, and the records are nearly
full rather than sparse. The live-byte histogram is 0:15, 2:1, 4:31, 5:238,
6:461, 7:1030, 8:1114.

That error also invalidated the reasoning built on it in this session's next
message, where the high zero-count was used to explain why the differential
analysis found no constant positions. There was no zero-density to explain.
The differential result stands -- all 8 positions vary -- but for the plain
reason that the records are dense, not because of any artifact.

New finding from re-examining it: byte 7 takes only 14 distinct values where
bytes 0-6 take 142-204. Its common values are 0x00 (1378), 0xff (806) and
0x80 (592), which are the high bytes of signed 16-bit values. That is
consistent with byte 7 being an MSB rather than ordinary data. Unexplained.

Also tried and recorded as not helping: a signed 16-bit reading spans
-32743..32633, near the full range, with no per-position structure (533, 371,
401 and 187 distinct at positions 0-3).

docs/formats/core-data-regions.md

@@ -368,15 +368,22 @@ statistics below. The matched-control autocorrelation puts the peak at stride
 **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.
+- **Mean 6.99 of 8 bytes are live** -- the previous revision of this file said
+  "zero", which is inverted and wrong. The live-byte histogram is 0:15, 2:1,
+  4:31, 5:238, 6:461, 7:1030, 8:1114, so most records are nearly full rather
+  than sparse.
 - **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.
+**Byte 7 is anomalous.** It takes only **14 distinct values** where bytes 0-6
+take 142-204. Its common values are `0x00` (1378), `0xff` (806) and `0x80`
+(592) -- the high bytes of signed 16-bit values, which is consistent with
+byte 7 being an MSB of something rather than ordinary data. Unexplained.
+
+**Not decoded.** A signed-16-bit reading was tried and does not obviously
+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.