re: the asteroid-field tables (AsteroidGroup_00N) decoded

New doc docs/re/structures/asteroid-fields.md, indexed.

Half the chain was already in the corpus: stage-definition-table and
stage-mission-tables record the per-phase AsteroidDefinition naming a .tbl,
and unit-datasheet-static partitions Generic and finds 10 per pack whose only
field is {EnumAsteroidGroup}, "an asteroid-group file". Neither says what is
inside them.

The table: 10 objects per pack x 6 packs = 60, 384 AsteroidGroup_* records,
4 or 7 groups per object. Every record carries exactly three fields, all 384,
no partials - AsteroidModelName (28 distinct; mob_n050..n056 x24 each,
mob_n040..n043 x18), AsteroidFrameName (64 distinct, Frame_S<NN>[_pN]_
Asteroid_NN, the same names regn-map-grid reports the Route tables showing),
and EnumAsteroid (37 distinct, 1..588, 6633 per pack summed).

Enum<Thing> is a COUNT; Enumerate<Thing> names a TABLE. EnumAsteroidGroup
equals its object's group count in 54 of 60 and EnumAsteroid is a plain
integer - the opposite of the Enumerate* join rule. The -erate is the
difference. Count-style fields still are not safe: the 6 failures are ONE
object repeated per language pack, declaring 4 while holding 7.

The tenth object is unreferenced and it is the one that miscounts. Joining by
frame prefix, nine objects match the nine AsteroidDefinition values one to
one; the tenth carries Frame_Alpha_S01_Asteroid, Frame_Alpha_S01_Haze and
Frame_TAlpha_S01_Asteroid and is named by nothing.

S28 has an asteroid VOLUME but no asteroid DEFINITION: mcol-collision lists
nine _AsteroidVolume_wp meshes (…, S28_p1); the nine AsteroidDefinition values
are those eight plus test_s8p1. Eight shared, one different each way.

Not settled: what EnumAsteroid counts of; why S28_p1 has a volume and no
definition. The name->object join did not reproduce by hash under name_hash,
mod 0x00FFF9D7, or a 24-bit mask - the join used is by frame prefix.

All fifteen artefacts byte-identical.

docs/re/INDEX.md

@@ -119,6 +119,7 @@ files, which is how the same ground got covered twice.
 | [`structures/isl-message-dialogue-link.md`](structures/isl-message-dialogue-link.md) | Mission scripts as dialogue — built-in 64 -> message id -> caption text | ✅ CONFIRMED total, 2 683/2 683 call sites across all 28 stages resolve, no residue |
 | [`structures/mission-objective-counter.md`](structures/mission-objective-counter.md) | `REMAINING OB` — the mission's own objective counter, in RAM |  ✅ CONFIRMED for one Stage 02 run: a big-endian u32 whose value |
 | [`structures/movie-subtitles.md`](structures/movie-subtitles.md) | Movie subtitles & the movie ↔ mission ↔ text chain | — |
+| [`structures/asteroid-fields.md`](structures/asteroid-fields.md) | `AsteroidGroup_00N` — the asteroid-field tables a `Phase_N.AsteroidDefinition` names | ✅ 10 objects/pack, 384 group records, 3 fields each (`AsteroidModelName`, `AsteroidFrameName`, `EnumAsteroid` = a count 1…588); `Enum<Thing>` counts while `Enumerate<Thing>` names a table (54/60); 🔑 a tenth `Frame_Alpha_S01_*` object nothing references — and it is the one that miscounts; 🔑 **S28_p1 has an asteroid volume but no definition** |
 | [`structures/mcol-collision.md`](structures/mcol-collision.md) | `MCOL` — same container and map parameters as `REGN` | 🟡 OPENED — `POF0` at `data_size+16` 11/11, bbox pad words 11/11, `extent == max−min` 11/11, and bbox + cell-size distributions identical to `REGN` (2/6/3 and 2/9); everything past `0x40` ❔ |
 | [`structures/regn-map-grid.md`](structures/regn-map-grid.md) | `REGN` — a per-map spatial grid (and `MCOL` beside it) | ✅ CONFIRMED for the header, which self-checks on all 11 objects on |
 | [`structures/savegame-format.md`](structures/savegame-format.md) | Save file (`savedata`) — container ✅ exact, 3 fields named ✅, rest ❔ (2026-08-11) | ✅ CONFIRMED for the container and the chunk layout — parsed off the |