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/testsuite/test.c

@@ -31,6 +31,7 @@ extern void test_wobj(void);
 extern void test_request(void);
 extern void test_zarray_cow(void);
 extern void test_kvmgr_capture(void);
+extern void test_hooks(void);
 
 void setUp(void) { return; }
 
@@ -131,6 +132,7 @@ int main(int argc, char* argv[])
     TEST(test_request);
     TEST(test_zarray_cow);
     TEST(test_kvmgr_capture);
+    TEST(test_hooks);
 
     return UNITY_END();
 }

src/lib/testsuite/test_hooks.c

@@ -0,0 +1,112 @@
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "hook.h"
+#include <unity/unity.h>
+
+/* Shared call counter for hook tests */
+static int s_call_count = 0;
+
+static int counting_fn(int type, int sit, int det, void* data, va_list ap)
+{
+    (void)type;
+    (void)sit;
+    (void)det;
+    (void)ap;
+    (*(int*)data)++;
+    return 0;
+}
+
+/* --- test cases --- */
+
+static void test_disable_hook_stops_invocation(void)
+{
+    hooksys_t hs;
+    int       rc;
+
+    s_call_count = 0;
+
+    rc = xce_hook_init(&hs, 2 /* maxtypes */);
+    TEST_ASSERT_EQUAL_INT(0, rc);
+
+    rc = xce_add_hook(&hs, HOOK_AFTER_EVENT, counting_fn, &s_call_count);
+    TEST_ASSERT_EQUAL_INT(0, rc);
+
+    /* Before disable: hook fires once */
+    xce_run_hooks(&hs, HOOK_AFTER_EVENT, 0, 0);
+    TEST_ASSERT_EQUAL_INT(1, s_call_count);
+
+    /* Disable the hook */
+    rc = xce_disable_hook(&hs, HOOK_AFTER_EVENT, counting_fn);
+    TEST_ASSERT_EQUAL_INT(0, rc);
+
+    /* After disable: hook must not fire */
+    xce_run_hooks(&hs, HOOK_AFTER_EVENT, 0, 0);
+    TEST_ASSERT_EQUAL_INT(1, s_call_count); /* unchanged */
+
+    xce_hook_fini(&hs);
+    printf("test_disable_hook_stops_invocation: PASS\n");
+}
+
+static void test_disable_hook_returns_enoent_if_not_found(void)
+{
+    hooksys_t hs;
+    int       rc;
+
+    rc = xce_hook_init(&hs, 2);
+    TEST_ASSERT_EQUAL_INT(0, rc);
+
+    /* Disabling a hook never added must return non-zero (ENOENT) */
+    rc = xce_disable_hook(&hs, HOOK_AFTER_EVENT, counting_fn);
+    TEST_ASSERT_NOT_EQUAL(0, rc);
+
+    xce_hook_fini(&hs);
+    printf("test_disable_hook_returns_enoent_if_not_found: PASS\n");
+}
+
+static void test_add_hook_reuses_null_slot(void)
+{
+    hooksys_t hs;
+    int       rc;
+    int       count_before_reuse;
+
+    s_call_count = 0;
+
+    rc = xce_hook_init(&hs, 2);
+    TEST_ASSERT_EQUAL_INT(0, rc);
+
+    rc = xce_add_hook(&hs, HOOK_AFTER_EVENT, counting_fn, &s_call_count);
+    TEST_ASSERT_EQUAL_INT(0, rc);
+
+    /* Record list length after first add */
+    count_before_reuse = hs.hooks[HOOK_AFTER_EVENT].count;
+    TEST_ASSERT_EQUAL_INT(1, count_before_reuse);
+
+    /* Disable the slot — fn becomes NULL atomically */
+    rc = xce_disable_hook(&hs, HOOK_AFTER_EVENT, counting_fn);
+    TEST_ASSERT_EQUAL_INT(0, rc);
+
+    /* Re-add the same fn — must reuse the NULL slot, not grow the list */
+    rc = xce_add_hook(&hs, HOOK_AFTER_EVENT, counting_fn, &s_call_count);
+    TEST_ASSERT_EQUAL_INT(0, rc);
+    TEST_ASSERT_EQUAL_INT(count_before_reuse, hs.hooks[HOOK_AFTER_EVENT].count);
+
+    /* Fire: must get exactly 1 call (not 2 if slot was duplicated) */
+    xce_run_hooks(&hs, HOOK_AFTER_EVENT, 0, 0);
+    TEST_ASSERT_EQUAL_INT(1, s_call_count);
+
+    xce_hook_fini(&hs);
+    printf("test_add_hook_reuses_null_slot: PASS\n");
+}
+
+/* --- suite entry point --- */
+
+void test_hooks(void)
+{
+    printf("\n>> hook system tests\n\n");
+
+    RUN_TEST(test_disable_hook_stops_invocation);
+    RUN_TEST(test_disable_hook_returns_enoent_if_not_found);
+    RUN_TEST(test_add_hook_reuses_null_slot);
+}