docs: editor coverage — version-2 B-trees, shrinking, dense attributes, reuse

CHANGELOG (Unreleased): the new FileEditor operations, space reuse, and
the two reader fixes (implicit index grid, object-header continuation
chains). known-issues: the editor's remaining refusals (skipped heap
blocks, heaps with filters or child indirect blocks, freeing a heap
block, implicit-index insertions, ...) and the append-waste sizes before
and after reuse (measure_append_waste, tank 2026-09-26; file sizes are
deterministic). range-reads design: status note on the reader changes.
README and CLAUDE.md: what the editor covers and how to test it.

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

CLAUDE.md

@@ -151,12 +151,18 @@ Cargo workspace with 18 crates under `crates/` (plus `libaec-sys`, an internal F
   `MemorySource` for this bookkeeping is inferred from the caller-supplied
   `source_channel` string (a heuristic, not an authenticated trust boundary).
 - In-place modification: `clawhdf5::FileEditor` (`crates/clawhdf5/src/edit/`)
-  overwrites values, grows chunked datasets and sets attributes in existing
-  files (h5py- or clawhdf5-written) without rewriting them; anything it
-  cannot do safely is `Error::Unsupported` before any write (limits in
+  overwrites values, grows and shrinks chunked datasets (every chunk index,
+  version-2 B-trees included) and sets attributes (compact and dense
+  storage) in existing files (h5py- or clawhdf5-written) without rewriting
+  them, changing indexes and heaps as libhdf5 does (index shapes and heap
+  bookkeeping are compared with libhdf5's in the tests); space an edit
+  frees is reused by later edits of the same editor. Anything it cannot do
+  safely is `Error::Unsupported` before any write (limits in
   `docs/known-issues.md`). Test changes with
-  `cargo test -p clawhdf5-tools --test edit_interop` (h5py, h5dump,
-  `h5rs check`).
+  `cargo test -p clawhdf5-tools --test edit_interop --test
+  edit_coverage_interop` (h5py, h5dump, `h5rs check`, structure comparisons
+  with libhdf5; libhdf5 sources for the algorithms are at
+  github.com/HDFGroup/hdf5, tag `hdf5_1_14_6`).
 - GPU-accelerated vector distance computation (`clawhdf5-gpu`, wgpu); HDF5 I/O itself is CPU-only
 - Browser: `clawhdf5-wasm` (wasm-bindgen, read-only, file held in memory;
   no Zstd/SZIP since they link C) and the `examples/wasm-viewer/` page.