feat: P3-6 ha_trigger_manual_failover_for_test() shim + 3 failover cycle tests

Add test-only function ha_trigger_manual_failover_for_test() to exercise manual
failover state machine (demote → promote cycle) without requiring live cluster_t.

Test coverage:
- test_manual_failover_transitions_role_and_increments_term: Validates role
  transitions (PRIMARY → STANDBY → PRIMARY) and term monotonicity
- test_manual_failover_resets_hooks_registered: Confirms demotion resets the
  hooks_registered idempotency guard
- test_manual_failover_monotonic_term_across_multiple_cycles: Validates term
  remains monotonically increasing across 5 consecutive failover cycles

Implementation:
- Exposed only under #ifdef XCE_TESTING in ha.h
- Function body (6 lines): calls ha_do_demote() then ha_do_promote(); returns
  the new term value via atomic_load_explicit()
- Enables design validation (P3-6) without test-cluster infrastructure

All tests pass. Build clean.

src/controller/testsuite/controller_test.c

@@ -29,6 +29,7 @@ extern void test_persist_repl(void);
 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);
 
 void setUp(void) { return; }
 
@@ -127,6 +128,7 @@ int main(int argc, char* argv[])
     TEST(test_repl_catchup);
     TEST(test_p2_integration);
     TEST(test_p3_hardening);
+    TEST(test_p3_manual_failover);
 
     return UNITY_END();
 }