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/meson.build

@@ -94,6 +94,7 @@ controller_unit_test_src = [
   'testsuite/test_repl_catchup.c',
   'testsuite/test_p2_integration.c',
   'testsuite/test_p3_hardening.c',
+  'testsuite/test_p3_manual_failover.c',
   'persist.c',
   'ha.c',
   'repl.c',
@@ -112,6 +113,7 @@ controller_unit_test = executable('controller_unit_test',
                                   controller_unit_test_src,
                                   include_directories: [incdir],
                                   dependencies: controller_unit_test_deps,
+                                  c_args: ['-DXCE_TESTING'],
                                   install: false)
 
 test('controller test', controller_unit_test,