Bound the last region by core_data's constant trailer

Comparing all 27 levels byte-for-byte from the end: the last 231,952 bytes
(0x38A10) of every level's core_data are identical. Level-specific content
therefore ends at len - 0x38A10, which is a real bound for the final region
and is not core_data.len().

This closes shrub. Across the 24 levels that have a shrub region the bound
gives 24,442,928 bytes total, and the disc test asserts on every level that
the region ends strictly inside the lump rather than at its end.

What the trailer is remains unestablished -- fixed data, or padding from a
fixed-size decompression buffer -- but it is constant, which is all a bound
needs.

The data between the last shrub class and the trailer is high entropy: 183 to
256 distinct byte values per 4 KB block, mean 254, where texture data sits
under 200. It is real content of unknown type and is the most likely home for
moby_anim, now that the range between collision and moby has been ruled out
as texture.

A lump shorter than the trailer now yields end: None rather than a backwards
bound, covered by a test.

crates/gc-extract/src/probe.rs

@@ -95,7 +95,7 @@ pub fn probe_level(dir: &Path, level: u32) -> Result<LevelProbe> {
         header.push((name.into(), range(v)));
     }
 
-    let regions = gc_formats::regions::core_layout(&index, &h);
+    let regions = gc_formats::regions::core_layout(&index, &h, core.len());
 
     let mut classes = Vec::new();
     for (name, r, stride) in [