test(ha-commit-engine): RED tests for per-LSN majority evaluation (task 1.1)

commit.h umbrella-verbatim interface + inert stub commit.c (test target
only); 7 Unity tests covering per-LSN vs acked_lsn>0 counting, majority
median, monotonicity, append clamp, lagging standby, RETAIN immediacy.
All 7 fail RED as intended. Documented 3 pre-existing masked suite
failures (bug-411) and the per-group exit-code masking root cause.

src/controller/meson.build

@@ -119,6 +119,8 @@ controller_unit_test_src = [
   'testsuite/test_ha_connection_reuse.c',
   'testsuite/test_ha_rdhup_integration.c',
   'testsuite/test_ha_liveness.c',
+  'testsuite/test_commit_engine.c',
+  'commit.c',
   'startup_validation.c',
   'worker.c',
   'persist.c',