feat: xce_disable_hook() + _Atomic hook_fn_t + NULL-slot reuse (P1-2a)

- hook_item_t.fn changed to _Atomic hook_fn_t (C11 stdatomic)
- xce_disable_hook(): atomically stores NULL into the slot fn field;
  returns 0 on success, -ENOENT if fn not found, -EINVAL on bad args
- xce_add_hook(): scans for NULL (disabled) slots before allocating;
  uses atomic_store_explicit(release) when writing fn pointer
- xce_run_hooks(): atomic_load_explicit(acquire) + NULL guard so
  disabled slots are silently skipped without disrupting the loop
- Add src/lib/testsuite/test_hooks.c with 3 TDD tests:
  test_disable_hook_stops_invocation
  test_disable_hook_returns_enoent_if_not_found
  test_add_hook_reuses_null_slot
- Register test_hooks in meson.build and test.c runner

Required by ha_demote_to_standby() to atomically disable persistence
hooks when the controller steps down from primary, preventing unbounded
hook list growth across promote/demote cycles (resolution A6-015,
A7-004, A7-016).

src/lib/meson.build

@@ -165,6 +165,7 @@ libxce_test_src = ['testsuite/test.c',
                    'testsuite/test_request.c',
                    'testsuite/test_zarray_cow.c',
                    'testsuite/test_kvmgr_capture.c',
+                   'testsuite/test_hooks.c',
                   ]
 
 libxce_test_deps = [libxce_deps, librtti_dep, unity_dep, libxce_dep]