Correct the chunk loader finding and decode a record field

The loader note said every record gains a 02 00 prefix in RAM. It does not:
the word is the record's own leading field, already on the disc at every
record. What the loader changes is the pointer -- it addresses the word, two
bytes earlier than the disc's table -- and that pointer shift is what made the
bytes look inserted.

Measured, in order:

- the two bytes before a payload are identical in core_data and in RAM, record
  for record: 02 00 on level 0's 90-record chunk at 0x100b302, 08 00 on the
  level's first chunk. Nothing is inserted;
- the RAM table's low 24 bits are the payload's minus 2, consistently, so the
  record is [leading u16][two index bytes][payload] and the loaded table
  addresses the leading word. That is also why the index bytes read as +2 from
  the pointer -- the same +2 the earlier RAM result quotes;
- the word is a field, not a pad: uniform across a chunk's records on 1,439 of
  1,509 chunks (None on the 70 that vary, reported rather than averaged),
  fixed within all 71 chunk types, and only ever 1, 2, 4, 6, 8 or 10 -- 554
  chunks carry 2, 520 carry 4.

Two readings of the value were measured and disproved, so they are recorded as
disproved rather than left as candidates: it is not an entry count (the payload
divided by it leaves 57 distinct fractional "sizes", and 162 chunks do not
divide at all), and it is not the number of distinct leading bytes (agrees on 0
of 1,506 chunks).

Chunk gains `prefix: Option<u16>`, and the disc test asserts the uniformity,
the per-type fixity and the value set. The scratch example that produced the
per-type class-ID table is dropped now that the same scan is a committed test,
which also lands here: moby class IDs sit in one body slot per chunk type, with
the slot varying by type and the distinct >= 4 filter load-bearing -- without it
the top slots are constants that read as 305x because any value below the moby
count counts as a hit.

docs/roadmap.md

@@ -352,11 +352,17 @@ Phase 1 hands over the extents; this phase works out what each one contains.
 - [x] **The loader's transformation, from PCSX2 RAM.** Disc offsets
       `0xf0000000 | low24` are rewritten in RAM to absolute EE pointers
       `0xf1850000 | low24` -- tag replaced by the chunk's load base, low 24 bits
-      untouched -- and every record gains a constant `02 00` prefix that the
-      rewritten table points at *instead of* the record data (**90 of 90**
-      records). The payload itself is copied **verbatim**. The index finding is
-      corroborated live: the byte at `+2` takes exactly `0, 16, ... 240`, the 16
-      values `index_rule` predicts.
+      untouched. The payload itself is copied **verbatim**. The index finding
+      is corroborated live: the byte at `+2` takes exactly `0, 16, ... 240`, the
+      16 values `index_rule` predicts.
+- [x] **Corrected: a record starts two bytes before the table's offset.** The
+      `02 00` read in RAM as a loader-inserted prefix is the record's own
+      leading word, already on the disc at every record. The loaded table
+      simply points two bytes earlier than the disc's, at that word. It is a
+      field: uniform across a chunk's records on **1,439 of 1,509** chunks,
+      fixed within **all 71 types**, values `{1, 2, 4, 6, 8, 10}`. Disproved
+      readings: an entry count (57 distinct fractional "sizes") and a
+      distinct-leading-byte count (0 of 1,506 agree).
 - [ ] The chunk **record fields are not decoded**, and no type is named. The body
       is dense (74% of `f32` reads plausible against a 49% random control) with
       an index-like tail -- 12 of 163 `u16` slots in the largest type stay below