feat: P3-7 ctrl_metrics — 7 atomic counters/gauges + descriptor table + persist wiring

Add operational metrics infrastructure for the XCE controller persistence
and HA subsystems (design doc P3-7, review A8-007).

- src/include/ctrl_metrics.h: declares 7 _Atomic uint64_t metrics with
  ctrl_ prefix, CTRL_METRIC_INC / CTRL_METRIC_SET macros, ctrl_metric_desc_t
  descriptor struct, ctrl_metric_descriptors[] table, and ctrl_metrics_init()
- src/controller/ctrl_metrics.c: defines the 7 counters/gauges, the
  descriptor table (7 entries + NULL sentinel), and ctrl_metrics_init()
- src/controller/persist.c: #include ctrl_metrics.h; call ctrl_metrics_init()
  at persist_start() entry; CTRL_METRIC_INC(ctrl_repl_degraded_count) when
  repl_wait_ack returns < 0 (degraded / local-only durability mode)
- src/controller/meson.build: add ctrl_metrics.c to xce_controller and
  controller_unit_test source lists; add test_ctrl_metrics.c to test sources
- src/controller/testsuite/test_ctrl_metrics.c: 5 TDD-first Unity tests
  covering init-zeros, increment, set/overwrite gauge, descriptor sentinel
  count, and transient-stuck counter accumulation — all PASS

src/controller/testsuite/controller_test.c

@@ -30,6 +30,7 @@ extern void test_repl_catchup(void);
 extern void test_p2_integration(void);
 extern void test_p3_hardening(void);
 extern void test_p3_manual_failover(void);
+extern void test_ctrl_metrics(void);
 
 void setUp(void) { return; }
 
@@ -129,6 +130,7 @@ int main(int argc, char* argv[])
     TEST(test_p2_integration);
     TEST(test_p3_hardening);
     TEST(test_p3_manual_failover);
+    TEST(test_ctrl_metrics);
 
     return UNITY_END();
 }