test(raft-drive-async): network-partition soak primitive + RED/GREEN scenario (task 4.1)

Add in-process partition blackhole primitive (no privilege required):
- Evict partitioned peer from standby_endpoints so reconnect fires
- RED mode: reconnect connect parks in gate → blocks delaymgr tick
  (proves pre-change build heartbeat stalls on unreachable peer)
- GREEN mode: real offload path runs → slowlane_submit non-blocking
  → tick returns promptly → survivor heartbeats continue every tick
- Heal: re-add peer, both standbys receive heartbeats post-heal

Requirements: 7.1, 7.2, 7.3, 7.4
Boundary: PartitionTest

Co-Authored-By: Claude <noreply@anthropic.com>

src/controller/meson.build

@@ -146,6 +146,7 @@ controller_unit_test_src = [
   'testsuite/test_ha_vep_teardown.c',
   'testsuite/test_ha_dispatch_integration.c',
   'testsuite/test_ha_core_safety.c',
+  'testsuite/test_ha_partition.c',
   'testsuite/test_waldump.c',
   'testsuite/waldump.c',
   'testsuite/test_walcompare.c',