refactor(ha): passive standby model — primary connects outbound to all peers

Remove fetch_config_from_primary, ha_find_and_connect_to_primary, and the
tier-1/2/3 discovery path. Standbys never initiate outbound connections to
the primary; the primary owns all inter-controller channels.

Key changes:
- ha.c: add primary heartbeat loop (ha_start_primary_heartbeat) that sends
  OPCode_nop to connected standbys every heartbeat_interval_ms; loop
  self-cancels on demotion via role check before re-arm.
- ha.c: add outbound reconnect in primary_heartbeat_fn — for peers absent
  from standby_endpoints, open fresh outbound TLS connection (on_standby_connected
  sends ctrlhello; standby_ep_ctx_t mirrors vote_ep_ctx_t layout).
- ha.c: remove ha_find_and_connect_to_primary + helpers (ha_raw_tcp_connect,
  ha_tcp_connect_hook); remove election_timeout_fn (superseded by
  raft_election_timeout_fn); remove fetch_config_from_primary call from
  ha_elect_evaluate_and_signal STANDBY branch.
- cluster.c: OPCode_requestVote denial no longer reuses inbound connection;
  late joiners are bootstrapped via primary heartbeat outbound connect.
- config_repl.c: remove connect_ssl_raw, do_one_config_fetch,
  fetch_config_from_primary (~250 LOC).
- myapp.c: remove my_tag > 0 fetch branch; all standbys wait passively.
- ha.h: remove ha_find_and_connect_to_primary declaration + macros +
  ha_tcp_connect_hook seam; add ha_start_primary_heartbeat declaration.
- meson.build: remove test_ha_connect.c (tests deleted function).
- controller_test.c: remove test_ha_connect registration.
- CLAUDE.md, src/controller/CLAUDE.md: document passive standby model,
  inbound-vs-outbound connection ownership, async I/O constraint,
  heartbeat model.
- request.h: add xce_request declaration (removes scattered local externs).

src/controller/meson.build

@@ -108,7 +108,6 @@ controller_unit_test_src = [
   'testsuite/test_ha_durability.c',
   'testsuite/test_repl_majority.c',
   'testsuite/test_persist_truncate.c',
-  'testsuite/test_ha_connect.c',
   'testsuite/test_ha_ctrlhello.c',
   'testsuite/test_ha_failover.c',
   'testsuite/test_ha_vote_send.c',
@@ -117,6 +116,7 @@ controller_unit_test_src = [
   'testsuite/test_ha_evaluate_committing.c',
   'testsuite/test_ha_repl_push.c',
   'testsuite/test_ha_repl_ack.c',
+  'testsuite/test_ha_connection_reuse.c',
   'startup_validation.c',
   'worker.c',
   'persist.c',