feat: implement persist_checkpoint with WAL marker, snapshot, trim, rotation (P0-3)

- persist_write_checkpoint_marker(): writes WAL_CHECKPOINT header entry
  with CRC, increments current_lsn; never updates last_replicated_crc
  (checkpoint markers are local-only, never replicated)
- write_snapshot(): atomic tmp→rename write of P0 sentinel snapshot
  ("XCESNAP\0" + checkpoint_lsn); full Cap'n Proto body in Task 11
- trim_wal_segments(): removes seg-*.wal files whose start LSN is
  strictly less than checkpoint_lsn
- rotate_snapshots(): collects .capnp files, sorts by name (LSN order),
  deletes oldest beyond snap_keep_n
- persist_checkpoint(): EBUSY guard via atomic_exchange, sequences
  marker→lsn record→snapshot→trim→rotate, clears flag on exit
- 5 TDD tests in test_persist_checkpoint.c covering: snapshot file
  creation, checkpoint_lsn advance, WAL lsn advance, EBUSY guard,
  and rotation keeping at most snap_keep_n files
- All 18 controller unit tests pass (0 failures)

src/controller/meson.build

@@ -71,6 +71,7 @@ controller_unit_test_src = [
   'testsuite/test_persist_types.c',
   'testsuite/test_persist_wal.c',
   'testsuite/test_persist_schema.c',
+  'testsuite/test_persist_checkpoint.c',
   'persist.c',
 ]