feat: add PersistConfig, SnapshotHeader, ReplEnvelope Cap'n Proto schemas (P0-9)

Extends src/schema/controller.capnp with persistence structures needed
for WAL replication and snapshots:

- PersistConfig (8 fields, @0-@7): capnp-serialisable copy of the
  persistence configuration; mapped to capnp_persist_cfg_t to avoid
  collision with the plain-C persist_config_t in persist.h
- SnapshotHeader (9 fields, @0-@8): snapshot manifest with LSN, term,
  timestamp, record counts and checksums; mapped to capnp_snap_hdr_t
- ReplEnvelope (6 fields, @0-@5): replication wire envelope (NOT the
  on-disk format); named to avoid collision with wal_entry_hdr_t and
  the WALEntry_ internal symbol c-capnproto generates; mapped to
  capnp_repl_envelope_t

Also extends existing structs:
- Job.transientSince @11 (UInt64=0, mapname transient_since): set when
  job enters JOB_STAT_TRANSIENT; also adds uint64_t transient_since to
  xce_job_t in types.h
- Worker.deadSince @6 (UInt64=0, mapname dead_since): mirrors existing
  time_t dead_since field in xce_worker_t

OPCode enum extended with 7 replication opcodes @22-@28:
  replEntry, replAck, redirect, heartbeat, steppingDown,
  replSnapshot, replCatchUpDone

New C typedefs (capnp_persist_cfg_t, capnp_snap_hdr_t,
capnp_repl_envelope_t) added to typemap.h for the encode/decode shims.

TDD: test_persist_schema.c added (6 tests, all GREEN). Controller test
suite remains 1/1 PASS.

src/controller/meson.build

@@ -70,6 +70,7 @@ controller_unit_test_src = [
   'testsuite/controller_test.c',
   'testsuite/test_persist_types.c',
   'testsuite/test_persist_wal.c',
+  'testsuite/test_persist_schema.c',
   'persist.c',
 ]
 
@@ -79,6 +80,7 @@ controller_unit_test_deps = [
   tinylog_dep,
   thread_dep,
   zlib_dep,
+  c_capnproto_dep,
 ]
 
 controller_unit_test = executable('controller_unit_test',