feat(agent): single-writer lock, read-only open, recoverable WAL

- HDF5Memory::create/open take an exclusive advisory lock on <store>.h5.lock
  (std File::try_lock, no new dependency). The store lives in memory and is
  rewritten wholesale at each checkpoint, so two handles on one store used to
  silently destroy each other's data; a second writer now gets
  MemoryError::Locked. The OS drops the lock with the descriptor, so a crash
  never leaves a stale lock. Acquisition retries for ~250 ms to absorb a
  previous owner that is mid-teardown; AsyncHDF5Memory::shutdown releases the
  lock once its writer task has stopped.
- HDF5Memory::open_read_only: a lock-free, point-in-time view (checkpoint +
  current WAL contents, replayed in memory) that never writes — it does not
  repair, upgrade or move the WAL, and anything that would persist returns an
  error. The CLI's recall/stats/agents-md/export use it, so a store can be
  inspected while an agent has it open. Tests that reopened a store purely to
  verify on-disk state now use it.
- open() no longer fails on a WAL that cannot possibly be replayed (torn
  header, bad magic): it is moved to <store>.h5.wal.corrupt-<ts>, reported via
  HDF5Memory::quarantined_wal(), and the healthy .h5 opens from its last
  checkpoint. A well-formed header with an unknown version still fails and is
  left untouched — most likely a newer build's WAL, which must not be
  discarded.

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

crates/clawhdf5-agent/src/async_memory.rs

@@ -408,6 +408,10 @@ impl AsyncHDF5Memory {
         let (tx, rx) = oneshot::channel();
         let _ = self.write_tx.send(WriteCmd::Shutdown(tx)).await;
         let _ = rx.await;
+        // The writer task has stopped, so nothing can write through this
+        // handle any more: release the single-writer lock now rather than at
+        // drop, so the store can be reopened while `self` is still in scope.
+        self.inner.lock().await.release_store_lock();
         Ok(())
     }
 }