test(ha-heartbeat-committed-propagation): verify vote floor consumes heartbeat-raised committed safely (task 3.2)

Test-only: exercises the real ha_vote_self_freshness/handle_request_vote gate.
A heartbeat raise via ha_observe_heartbeat_committed lifts the floor to
max(durable=5, raised=9); candidate at 7 denied, at 9/12 granted; a lower
heartbeat never lowers it. Discriminating constants fail under either regression
(floor->durable-only or fold->min). No production/vote-path change (Req 3.2).
123/0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/controller/meson.build

@@ -135,6 +135,7 @@ controller_unit_test_src = [
   'testsuite/test_commit_durability.c',
   'testsuite/test_ctrlstatus.c',
   'testsuite/test_ha_heartbeat_committed.c',
+  'testsuite/test_ha_vote_floor_heartbeat.c',
   'ctrlstatus.c',
   'commit.c',
   'job.c',