feat: persistence hook functions wired to WAL write on AFTER_EVENT (P0-6)

Implements persist_job_hook_fun, persist_queue_hook_fun, and
persist_worker_hook_fun — AFTER_EVENT hooks that write WAL entries
whenever state mutations occur in the controller.

- detail_to_wal_type() maps all 15 HOOK_DETAIL_* values to WAL types;
  unknown details return 0 and are silently skipped
- HOOK_DETAIL_JOB_NEW uses the sync replication path (WAL_JOB_SUBMIT);
  all others use persist_wal_async
- P0 payload: 4-byte detail value; full Cap'n Proto serialization in Task 15
- Hook signature matches hook_fn_t exactly:
  int fn(int type, int situation, int detail, void* cls, va_list ap)
- Added <stdarg.h> to persist.h for va_list; added hook.h and
  xce/cluster_def.h includes to persist.c
- 18 new unit tests in test_persist_hooks.c covering all 15 detail→WAL
  mappings, skip-on-unknown, and sequential LSN advancement

src/include/persist.h

@@ -1,5 +1,6 @@
 #pragma once
 #include <pthread.h>
+#include <stdarg.h>
 #include <stdatomic.h>
 #include <stddef.h>
 #include <stdint.h>
@@ -145,3 +146,20 @@ int persist_checkpoint(persist_t* p);
 
 /* Logging helper — never logs payload content (security invariant) */
 void persist_log_entry_metadata(const wal_entry_hdr_t* hdr);
+
+/* ============================================================
+ * AFTER_EVENT hook functions — registered by persist_start()
+ *
+ * Signature matches hook_fn_t: int fn(int type, int situation,
+ *                                     int detail, void* data, va_list ap)
+ * cls/data points to the persist_t instance.
+ *
+ * P0: writes a minimal 4-byte payload (detail value) to the WAL.
+ * Full Cap'n Proto serialization is wired in Task 15.
+ * ============================================================ */
+int persist_job_hook_fun(int type, int situation, int detail,
+                         void* cls, va_list ap);
+int persist_queue_hook_fun(int type, int situation, int detail,
+                           void* cls, va_list ap);
+int persist_worker_hook_fun(int type, int situation, int detail,
+                            void* cls, va_list ap);