Identify data.bin as the level file descriptor, not geometry

data.bin is the largest per-level file -- 13.3 MB on level 0, 4.2 to 21.9 MB
across the disc -- and had been characterised only as "4-byte structured".

Range measured: the first 64 u32 of data.bin on levels 0, 1, 2 and 5, compared
against the actual byte lengths of the other extracted level files.

Method: read the first 64 u32 and look for any equal to the size of
core_index.bin, gs_ram.bin, overlay.bin, gameplay.bin, sound_bank.bin or
core_data.bin.

Result: three exact matches, at the same indices on every level.

  u32 index 1  overlay.bin     2751268  2763668  2928424  2863676
  u32 index 3  core_index.bin    31600    34016    33808    23536
  u32 index 5  gs_ram.bin       518656   569344   513024   434176

Each is the exact byte length of the named file and each lands on the same index
on all four levels. Three coincidences at fixed positions is not a coincidence.

So data.bin is a descriptor for the level's other files, not geometry in its own
right. The roadmap's "the large one: 4.2 to 21.9 MB of geometry per level" entry
was wrong and is corrected.

The intervening indices 0, 2 and 4 hold values that are not file sizes and their
meaning is not established; they are plausibly offsets, but no cumulative-sum or
offset+size relation has been checked yet.

What follows the descriptor is still unknown, and the file is large enough that
the descriptor is unlikely to be all of it.

docs/roadmap.md

@@ -217,7 +217,10 @@ Phase 1 hands over the extents; this phase works out what each one contains.
       what it looked like. Corrected in `plan/level-assets.md`.
 - [ ] `occlusion` — 2 to 25 sectors, small enough to read whole.
 - [ ] `sound_bank` — `SBlk` and `DAW` signatures found in all 27.
-- [ ] `data` — the large one: 4.2 to 21.9 MB of geometry per level.
+- [x] `data` — **not geometry.** It is the level's file descriptor: its
+      first table carries the exact byte lengths of `overlay.bin`,
+      `core_index.bin` and `gs_ram.bin` at fixed `u32` indices 1, 3 and 5 on
+      every level checked. What follows the descriptor is still unknown.
 
 - [ ] `tfrag` — terrain: LOD chains, strips, morph targets
 - [ ] `tie` — instanced environment geometry, fat vertices