docs: changelog and CLAUDE.md for the durability & integrity work

Co-Authored-By: Claude Fable 5.1 <[email protected]>

CLAUDE.md

@@ -40,6 +40,25 @@ Cargo workspace with 16 crates under `crates/` (plus `libaec-sys`, an internal F
   format (v2) is still fully readable; the oldest no-CRC format (v1) is only
   reachable through the one-time migration path in `HDF5Memory::open`, not
   through the public `WalFile::read_entries`.
+  **What the WAL guarantees:** integrity, ordering, and recovery from a
+  *process* crash at any point — including between a checkpoint and the WAL
+  truncate (each checkpoint records a `WalMark` in `/meta`, and `open()` skips
+  the WAL prefix the `.h5` already contains, so entries are never applied
+  twice). Checkpoints and snapshots are made durable as a unit (temp file
+  synced, renamed, directory synced). **What it does not guarantee:**
+  individual WAL appends are *not* fsynced (a deliberate latency trade-off), so
+  saves made since the last checkpoint can be lost on power failure or kernel
+  panic. Current header version is 4 (adds the `Update` record used by
+  `save_or_update`); v3 files are read and upgraded in place.
+- A store has a **single writer**: `HDF5Memory::create`/`open` hold an exclusive
+  advisory lock on `<store>.h5.lock` and a second opener gets
+  `MemoryError::Locked`. Use `HDF5Memory::open_read_only` for a lock-free,
+  never-writing point-in-time view (the CLI's `recall`/`stats`/`agents-md`/
+  `export` do). An unreadable WAL (torn header, bad magic) is quarantined to
+  `<store>.h5.wal.corrupt-<ts>` rather than blocking `open()`; a WAL with an
+  unknown *newer* version still fails and is left untouched.
+- `MemoryConfig::compression` uses deflate by default; enable the agent's
+  `zstd` feature to compress embeddings with Zstd instead (links libzstd).
 - `Dataset::verify_provenance()` (clawhdf5 facade, `provenance` feature, on by
   default) recomputes a dataset's SHA-256 and compares it against the
   `_provenance_sha256` attribute written automatically on save when