Resolve the shrub region bound: the chunk container closes it

The shrub region's end was the last unlocated bound in core_data. It was not
in the core index, and unlike moby->tie it had no successor boundary to chain
against, so the last shrub class in every level ran out to the lump's end and
the aggregates over it (the "3.5M records / 28MB of shrub body") were
artifacts of that oversized span.

Range measured: for each level, from the last shrub class's geometry offset to
the constant trailer. Nothing outside that range is read.

Method: scan forward from the last shrub class for the pair (a) the constant
two bytes f6 7c sitting immediately in front of (b) a chunk signature -- a
16-byte header ending 0xff, ten zero bytes, then four or more 0xf0-tagged
offsets. The two halves are each weak alone, and the halves are measured
separately: the marker pair occurs 25 to 103 times in a 5 to 17 MB lump, so
matching it alone picks dozens of wrong offsets; the chunk shape matches
random data often enough to need four tagged offsets before it is believed.

Result: on all 27 levels the marker sits exactly two bytes before the first
chunk header, and the two parts of the signature are never found apart. No
chunk signature occurs inside any last shrub body, on any level. The three
levels without shrub (10, 25, 26) carry the same marker before the container
that closes tie instead, which is what makes the locator a property of the
region rather than a shrub rule.

Consequences:

- All 358 shrub class bodies are bounded and are whole 8-byte records. That
  was not assertable while the last class in each level ran to the lump's end,
  and it is the check the boundary earns its keep for.
- 9,600 records in total, against the 8,938 previously measurable in
  successor-bounded spans; 662 of them sit in the 24 last-class bodies. The
  shrub region totals 2,033,776 bytes across the 24 levels that have one.
- The chunk container is 25,775,850 bytes across 27 levels (1,236 chunks,
  22,562 records). The container measurements were previously taken over "the
  900,000 bytes before the trailer", a window that did not align to its real
  extent: on level 0 it is 690,302 bytes, and 27 of 1,123 chunks were
  truncated at that invented edge. chunk_containers_resolve_on_every_level now
  reads the region from core_layout instead of measuring backwards from the
  end of the lump.

core_layout takes core_data now: the container's start is the one bound that
cannot be derived from the index. It gains the chunks region, so every byte of
level-specific core_data belongs to a named region. Where the signature is not
found the end stays None -- the region is left visible rather than clamped to
the tail.

Still open: the marker's meaning (constant across 27 levels, at exactly this
position, unnamed) and the shrub body payload's encoding.

docs/formats/core-data-regions.md

@@ -27,8 +27,10 @@ than inventing their own.
 | `sky`      | `sky`          | `collision`          |
 | `collision`| `collision`    | first moby class     |
 | `moby`     | first moby     | first tie class      |
-| `tie`      | first tie      | first shrub class    |
-| `shrub`    | first shrub    | lump length − 0x38A10 |
+| `tie`      | first tie      | first shrub class,   |
+|            |                | else chunk container |
+| `shrub`    | first shrub    | chunk container      |
+| `chunks`   | chunk container| lump length − 0x38A10|
 
 Class regions start at the lowest non-zero geometry offset in their class
 table. A zero entry means *absent*, not position zero -- the same trap the
@@ -43,26 +45,79 @@ Comparing all 27 levels byte-for-byte from the end: the last **231,952 bytes
 therefore ends at `len - 0x38A10`, and the final region is bounded there rather
 than at `core_data.len()`.
 
-This is what closes shrub. Across the 24 levels that have a shrub region it
-gives **24,442,928 bytes** total, and it is the same bound the disc test
-asserts on every level.
+This is what closes the chunk container. The regions it separates from level
+content are measured on the disc: **2,033,776 bytes of shrub region across the
+24 levels that have one, and 25,775,850 bytes of chunk container** across all
+27. The disc test asserts the bound on every level.
 
 What the trailer *is* is not established. It may be fixed data, or padding
 from a fixed-size decompression buffer. Either way it is constant, which is
 all that is needed to bound the tail.
 
-## Open: what was in the region before it
+## Found: the chunk container closes the class region
 
-The offset ending the shrub region is not in the core index.
+The offset ending the shrub region is not in the core index, and the class
+table that precedes the container carries no successor offset for it -- unlike
+moby and tie, whose boundaries fall out of the tables. Texture texels do not
+bound it either: they sit before moby, not after shrub. So the boundary is
+located in `core_data` itself.
 
-Texture texels do **not** bound it. They sit before moby, not after shrub: on
-level 0 the texel range is 3539328..5326208 while the last shrub class starts
-at 16044288, leaving ~925 KB of real, non-zero data after the last shrub class
-that nothing yet accounts for.
+**Range measured:** for each level, from the last shrub class's geometry
+offset to the constant trailer. Nothing outside that range is read.
 
-`core_layout` reports `end: None` for shrub on the 24 levels that have one, and
-the disc test asserts it, so the gap stays visible rather than being papered
-over by clamping to the end of the lump.
+**Method:** scan forward from the last shrub class for the pair (a) the
+constant two bytes `f6 7c`, sitting immediately in front of (b) a chunk
+signature -- a 16-byte header ending `0xff`, ten zero bytes, then four or more
+`0xf0`-tagged offsets. `gc_formats::chunks::chunk_region_start`.
+
+Each half of that signature is weak on its own, which is why the halves are
+measured separately:
+
+- the pair `f6 7c` occurs **25 to 103 times** in a 5 to 17 MB lump, so
+  matching it alone picks dozens of wrong offsets;
+- the chunk shape matches random data often enough that `parse_chunk` demands
+  four tagged offsets before a signature is believed.
+
+**Result: on all 27 levels the pair sits exactly two bytes before the first
+chunk header.** The two are never found apart. This is the boundary, and
+`core_layout` closes shrub with it -- `shrub`'s end is the container's start,
+and the container's own end is the trailer, so every byte of level-specific
+`core_data` now belongs to a named region.
+
+Two controls, both asserted by the disc test `chunk_container_closes_the_last_class_region`:
+
+- **No signature inside the body.** Scanning the last shrub body for a bare
+  chunk signature finds none, on any level -- so the boundary search was not
+  matching something it should have skipped, and the marker's job is
+  corroboration rather than load-bearing disambiguation.
+- **The three levels without shrub** (10, 25, 26) carry the same marker two
+  bytes before the container. There the container closes *tie* instead, which
+  is what makes the locator a property of the region rather than a shrub
+  rule.
+
+On level 0:
+
+| what | range | bytes |
+|---|---|---|
+| last shrub class data | 16046800 .. 16047040 | 240 |
+| marker `f6 7c` | 16047040 .. 16047042 | 2 |
+| **chunk container** | 16047042 .. 16737344 | **690,302** |
+| constant trailer | 16737344 .. 16969296 | 231,952 |
+
+The marker's meaning is not established. It is constant across all 27 levels
+and it appears at exactly this position, but whether it is a terminator, an end
+count or a tag is unknown, so it is not named.
+
+**Correction this produces.** The chunk-container measurements recorded below
+were taken over "the 900,000 bytes before the constant trailer" -- a window
+wide enough to contain the container on every level but not aligned to its
+true extent. Chunks sitting at the window's edge were truncated (27 of 1,123),
+and the region's size was never correctly stated. Both are now bounded
+properly: on level 0 the container is 690,302 bytes, not ~900,000, and the
+25,775,850 bytes of chunk data across 27 levels are the measured total rather
+than a windowed sample. The `chunk_containers_resolve_on_every_level` test now
+reads the region from `core_layout` instead of measuring backwards from the
+end of the lump.
 
 ## Correction: it is mostly texture data, not an unnamed region