docs: FileEditor — changelog, limits and leaked space, README example

known-issues records what the editor refuses, that freed space is never
reused (append-workload file sizes measured 2026-09-26 on tank with the
ignored measure_append_waste test; sizes are deterministic), and that there
is no journal.

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

CLAUDE.md

@@ -150,6 +150,13 @@ Cargo workspace with 18 crates under `crates/` (plus `libaec-sys`, an internal F
   Alerts never block a save — drain them with `HDF5Memory::take_anomaly_alerts`.
   `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
+  `docs/known-issues.md`). Test changes with
+  `cargo test -p clawhdf5-tools --test edit_interop` (h5py, h5dump,
+  `h5rs check`).
 - 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.