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-cli/src/main.rs

@@ -146,7 +146,7 @@ fn run(cli: Cli) -> Result<(), Box<dyn std::error::Error>> {
         }
 
         Commands::Recall { index } => {
-            let mem = HDF5Memory::open(&cli.path)?;
+            let mem = HDF5Memory::open_read_only(&cli.path)?;
             match mem.get_chunk(index) {
                 Some(content) => {
                     let j = serde_json::json!({ "index": index, "chunk": content });
@@ -160,7 +160,7 @@ fn run(cli: Cli) -> Result<(), Box<dyn std::error::Error>> {
         }
 
         Commands::Stats => {
-            let mem = HDF5Memory::open(&cli.path)?;
+            let mem = HDF5Memory::open_read_only(&cli.path)?;
             let cfg = mem.config();
             let j = serde_json::json!({
                 "path": cli.path.display().to_string(),
@@ -187,7 +187,7 @@ fn run(cli: Cli) -> Result<(), Box<dyn std::error::Error>> {
         }
 
         Commands::AgentsMd { output } => {
-            let mem = HDF5Memory::open(&cli.path)?;
+            let mem = HDF5Memory::open_read_only(&cli.path)?;
             let md = mem.generate_agents_md();
             match output {
                 Some(p) => {
@@ -199,7 +199,7 @@ fn run(cli: Cli) -> Result<(), Box<dyn std::error::Error>> {
         }
 
         Commands::Export => {
-            let mem = HDF5Memory::open(&cli.path)?;
+            let mem = HDF5Memory::open_read_only(&cli.path)?;
             for i in 0..mem.count() {
                 if let Some(chunk) = mem.get_chunk(i) {
                     let j = serde_json::json!({ "index": i, "chunk": chunk });