docs: M1 changelog after review: probe identity, speed, bounded reads

- Per-file probe output is identical for 696 of 697 files, not all:
  cve-2025-2310.h5's error string depends on which parallel chunk decode
  fails first, at f2ff2c4 as on this branch.
- The parser cores are generic (S: Storage + ?Sized); provisional A/B
  numbers against f2ff2c4, including the one bench that still shows
  ObjectHeader::parse slower when old and new are separate binaries.
- Reads sized by untrusted fields are bounded; the harness only accepts
  the known whole-file fallbacks.
- range-reads.md records why M1 went generic rather than &dyn.

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

CHANGELOG.md

@@ -16,8 +16,11 @@
   and (with `std`) `Arc`s of a `Storage`. Slices and `Vec`s serve borrowed
   bytes, so parsing an in-memory file costs no copy.
 - **The metadata parsers read through `Storage`.** Each converted parser has
-  an `*_in(&dyn Storage, ..)` core, and its `&[u8]` function is now a thin
-  wrapper over it, so no caller changes: the superblock
+  an `*_in<S: Storage + ?Sized>(&S, ..)` core (a `&dyn Storage` works too),
+  and its `&[u8]` function is now a thin wrapper over it, so no caller
+  changes. The wrappers compile to a `[u8]` instance of the same code, so a
+  structure read in memory is a bounds check and a borrowed slice, with no
+  indirect call and no copy. Converted: the superblock
   (`Superblock::parse_in`), its extension and cache image
   (`read_superblock_extension_in`, `cache_image_state_in`), object headers
   with their continuation chunks (`ObjectHeader::parse_in`), local and
@@ -32,6 +35,17 @@
   (a prefix, then the structure) instead of slicing the whole file; the
   open-ended `&file_data[addr..]` slices in these modules are gone. Bounds
   errors keep their values (absolute position, file length).
+- Reads sized by untrusted fields are bounded by what the parser uses, so
+  a crafted size cannot turn one structure into a read of the rest of the
+  file on a range backend: local-heap names are read in growing pieces
+  (64 bytes first) rather than to the end of the data segment; a fractal
+  heap indirect block is read up to the entry covering the object (the
+  whole block only when that entry is unallocated); paged fixed and
+  extensible array data blocks over 1 MiB are read page by page, only the
+  pages in use; and a block under one checksum whose claimed size runs
+  past the end of the file fails its bounds check before any read (with
+  the `checksum` feature). An object header's prefix is one read (was
+  two).
 - Structures still indexed by a v2 B-tree — dense attribute storage, a SOHM
   B-tree index and huge fractal-heap objects found through their B-tree —
   are not converted yet (the
@@ -42,11 +56,27 @@
 - **No behaviour change**, checked three ways (2026-09-26, tank): every
   existing test passes unchanged; the conformance sweep
   (`conformance/run.sh --no-fetch`) gives a byte-identical `results.json`
-  and identical per-file probe output for all 697 files at `f2ff2c4` and on
-  this branch; and a transcript of every converted `&[u8]` function's
-  result over the fixtures, the conformance corpus and the h5py-written
-  files below (748 files, 7 603 object headers) is byte-identical between
-  the two builds.
+  at `f2ff2c4` and on this branch, and identical per-file probe output for
+  696 of the 697 files — the exception, `cve-2025-2310.h5`, reports one of
+  two errors depending on which parallel chunk decode fails first, at
+  `f2ff2c4` as on this branch; and a transcript of every converted `&[u8]`
+  function's result over the fixtures, the conformance corpus and the
+  h5py-written files below (748 files, 7 603 object headers) is
+  byte-identical between the two builds.
+- **Speed on local files** (provisional: tank was shared with other jobs;
+  both builds linked into one binary and timed alternately, 200 rounds;
+  new Criterion bench `clawhdf5/benches/local_metadata_bench.rs` over a
+  400-group version-1 file, `clawhdf5-format/tests/fixtures/v1_groups_400.h5`):
+  against `f2ff2c4`, listing the file through the facade is 2.7% faster,
+  `ObjectHeader::parse` is within ±1%, symbol-table nodes and the group
+  B-tree walk are about 19% faster (their entry loops were tightened),
+  local-heap names and `resolve_group_children` 1.5–3% faster. The same
+  harness run on two copies of the old code differs by up to 2%. The
+  Criterion bench itself, old and new as separate binaries run alternately
+  (3 rounds), agrees except for `ObjectHeader::parse`, which it puts about
+  7% slower (25.9 vs 24.1 µs for 401 headers) while the listing that
+  parses those headers is 5–8% faster; that one remains unexplained and is
+  to be rechecked on an idle machine.
 - New equivalence harness `clawhdf5-format/tests/storage_equivalence.rs`:
   every converted parser runs over the file as a slice and over
   `storage::CountingStorage` — a `Storage` that serves an in-memory buffer
@@ -55,8 +85,12 @@
   the fixtures, files h5py writes for it (extensible arrays with super
   blocks and paged data blocks, paged fixed arrays, large v1 and dense
   groups, a user block, SOHM list and B-tree indexes, dense, shared and
-  committed-type attributes), and with `CLAWHDF5_STORAGE_CORPUS=<dir>` a
-  corpus (all 653 HDF5 files of the conformance corpus pass). Milestones M2
+  committed-type attributes, fixed and extensible array data blocks over
+  1 MiB, whole and truncated), and with `CLAWHDF5_STORAGE_CORPUS=<dir>` a
+  corpus (all 653 HDF5 files of the conformance corpus pass). Only the
+  structures listed above as not converted may answer
+  `ContiguousStorageRequired`, and only in the checks that reach them; a
+  converted parser falling back to the whole file fails it. Milestones M2
   and M3 extend it.
 
 ### Chunked full reads (2026-09-26)