test(format): fuzz Blosc2 decoding for peak allocation

The Blosc2 fuzz tests checked only for panics and the size of the output,
so the offsets-chunk amplification passed all 40,000 iterations.
tests/blosc2_alloc_bounds.rs now measures peak allocation (a counting
global allocator) and asserts it stays within 6x the HDF5 chunk size plus
twice the input plus 2 MiB (ruzstd's fixed state) for:

- 20,000 mutated fixture frames, decoded with their real chunk size as
  the limit, with edits aimed at the frame's and chunks' size fields;
- 20,000 mutated first chunks on their own;
- 5,000 frames built from random header sizes, offsets chunks and B2ND
  shapes (padding chunk and block shapes, chunks larger than the array,
  NaN, zero and repeated-value chunks).

Against the code before this series every test in the file fails (the
fuzz tests at frame iteration 3913, a zstd window, and random frame 289,
the offsets chunk); now the worst frame peaks at 0.48 of the bound.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-format/src/filters_blosc2.rs

@@ -1246,7 +1246,8 @@ mod tests {
     }
 
     /// Random and mutated frames: errors are fine, panics are not, and no
-    /// output past the limit.
+    /// output past the limit. Peak allocation is fuzzed separately, with a
+    /// counting allocator, in `tests/blosc2_alloc_bounds.rs`.
     #[test]
     fn fuzzed_frames_never_panic() {
         let limit = 20_000;