feat(agent): Ed25519-signed checkpoints

Makes the README's "cryptographically verifiable memory" true.

With HDF5Memory::set_signing_key(key), every checkpoint stores a signed
manifest of the store: a SHA-256 per memory record (text, embedding as
stored, channel, timestamp, session, tags, deleted flag, activation) in
a Merkle tree, plus hashes of the settings (and WAL mark), sessions and
knowledge graph. The signature, public key and manifest hashes go in
/meta; the per-record hashes in /integrity/record_hashes, so
HDF5Memory::verify(path, &public_key) can say which records changed, not
just that something did. A forged manifest fails the signature.

Decisions, as agreed:
- the key is set on the open store and never persisted;
- a signed store refuses to checkpoint without its key
  (MemoryError::SigningKeyRequired); remove_signature() is the
  deliberate way back to unsigned;
- checkpoints only: saves still in the WAL are not covered, and verify
  reports how many there are.

The hashes cover exactly what the file persists, in the form the loader
returns it (strings lose trailing NULs; an empty WAL mark is not
written), so untouched stores verify across any number of reopen and
checkpoint cycles. MemoryError becomes #[non_exhaustive] (it already
gains variants in this unreleased version).

CLI: keygen (owner-only key file), --signing-key / CLAWHDF5_SIGNING_KEY
on writing commands (create signs immediately), verify --public-key
(JSON; exit 2 if not valid), `signed` in create/stats output.

Tests: reopen/checkpoint cycles with awkward strings (f16 and f32),
refusal without the key, wrong and rotated keys, eight kinds of edit
each detected and located, a forged manifest, unsigned stores, NULs in
text, and an edit made in place with h5py that verify pinpoints.

Cost on tank (search_harness --signing-study --full, 3 runs): ~20% of a
checkpoint (+9 ms at 10K, +89-112 ms at 100K), verify 18.6 ms / 247 ms,
32 bytes per record in the file. New deps ed25519-dalek, sha2,
rand_core: pure Rust, the no-C check passes.

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

CLAUDE.md

@@ -109,6 +109,17 @@ Cargo workspace with 16 crates under `crates/` (plus `libaec-sys`, an internal F
   `search_harness --options-study`.
 - `MemoryConfig::compression` is off by default; when on, embeddings are
   deflate-compressed, or Zstd with the agent's `zstd` feature (links libzstd).
+- Signed checkpoints (`clawhdf5-agent` `signing` module): with
+  `HDF5Memory::set_signing_key` every checkpoint stores an Ed25519-signed
+  manifest (SHA-256 per record in a Merkle tree + settings/sessions/graph
+  hashes; per-record hashes in `/integrity/record_hashes`);
+  `HDF5Memory::verify(path, &pk)` locates edits. The hashes must cover exactly
+  what the file persists in the form the loader returns it (strings lose
+  trailing NULs; an empty WAL mark is not written) or untouched stores stop
+  verifying — `tests/signed_store.rs` round-trips awkward strings. The key is
+  never persisted; a signed store refuses to checkpoint without it
+  (`MemoryError::SigningKeyRequired`, and `MemoryError` is `#[non_exhaustive]`).
+  WAL entries after the checkpoint are not covered.
 - `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