feat: implement persist_recover with snapshot load and WAL replay (P0-4)

- Add load_snapshot(): reads P0 sentinel ("XCESNAP\0" + lsn), validates magic,
  extracts checkpoint_lsn
- Add replay_wal(): scans seg-*.wal files in LSN order, skips entries <= from_lsn,
  verifies CRC chain, stops on mismatch, applies entries via apply_wal_entry() stub
- Add persist_recover(): find latest .capnp snapshot, load it, replay post-checkpoint
  WAL, retention pass (no-op in P0)
- Fix trim_wal_segments(): compare against active segment start LSN (not checkpoint_lsn)
  to avoid deleting the currently-open segment and losing post-checkpoint WAL entries
- Add restore_job() stub in job.c (P0-12a placeholder)
- Add worker_from_capnp() stub in worker.c (P0-12 placeholder)
- Declare both new functions in cluster.h and worker.h
- Add 5 recovery tests: empty state, snapshot lsn loading, WAL replay, corrupt
  sentinel rejection, latest-snapshot selection; all 23 controller tests pass

src/controller/meson.build

@@ -72,6 +72,7 @@ controller_unit_test_src = [
   'testsuite/test_persist_wal.c',
   'testsuite/test_persist_schema.c',
   'testsuite/test_persist_checkpoint.c',
+  'testsuite/test_persist_recovery.c',
   'persist.c',
 ]