feat: P2-2 apply_wal_entry — standby WAL replay with LSN advance and type guards

Upgrade apply_wal_entry() from P0 static no-op stub to the real public
standby replay path:

- Reject oversized payloads (> MAX_WAL_PAYLOAD_BYTES) with -EMSGSIZE
- Skip WAL_CHECKPOINT (100) entries silently — local-only markers, never
  replicated to standbys; current_lsn is NOT advanced on skip
- Skip unknown types (> WAL_ENTRY_TYPE_MAX) with WARN log and rc=0 for
  forward compatibility with WAL written by newer controller versions
- Advance current_lsn to hdr->lsn+1 and update last_crc under p->lock
  for all accepted known entries
- Gate restore_job()/worker_from_capnp() calls on p->cluster != NULL so
  unit tests work without a full cluster wired in
- Add NULL guard returning -EINVAL for missing p or hdr

Add public declaration in persist.h with forward-declare stubs for
restore_job/worker_from_capnp in persist.c (avoids heavy cluster.h chain).

Test suite: 5 new Unity tests in test_repl_apply.c covering all branches.
Weak stubs for restore_job/worker_from_capnp in the test file satisfy the
linker without linking job.c/worker.c's heavy dependency graph.
All 5 new tests pass; full controller suite green.

src/controller/testsuite/controller_test.c

@@ -24,6 +24,7 @@ extern void test_ha_cluster(void);
 extern void test_ha_redirect(void);
 extern void test_p1_integration(void);
 extern void test_repl_unit(void);
+extern void test_repl_apply(void);
 
 void setUp(void) { return; }
 
@@ -117,6 +118,7 @@ int main(int argc, char* argv[])
     TEST(test_ha_redirect);
     TEST(test_p1_integration);
     TEST(test_repl_unit);
+    TEST(test_repl_apply);
 
     return UNITY_END();
 }