- 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).
