fix(ha): bug-1421 close term-desync AE-reject blindspot in HA election

A primary never learns of a follower's higher term via AppendEntries-reject
responses: ha_raft_on_repl_ack only routes AE replies through
raft_on_append_entries_reply for success=1 ACKs, and that function itself
drops any reply whose term != s->term (raft.c stale-term drop) before it
could adopt a higher term anyway. Net effect: once a follower outpaces a
stale primary's term, every AppendEntries the primary sends is rejected
forever and it can never regain write-quorum — a permanent election livelock
(surfaced as "no primary elected at term>8" in HA soak testing).

Adds two fallback term-discovery vectors that fire regardless of AE-reject
outcome:
- StateReport heartbeat-reply now carries the standby's current term
  (controller.capnp, typemap.h) so the primary learns a higher term on every
  convergence cycle.
- New raft_on_higher_term_observed() mirrors the UpdateTerm step (strictly
  greater-than only; touches term/voted_for/role; persists before notifying
  on_become_follower) and is wired into both the heartbeat state-report path
  and the WAL-NACK path in repl_ack_decode_and_dispatch.

Validated: 265 controller + 287 lib unit tests pass; 10-round sequential and
chaotic HA soaks both PASS with zero job loss and full leadership coverage.

src/schema/controller.capnp

@@ -818,6 +818,7 @@ struct ReplEnvelope $C.mapname("capnp_repl_envelope_t") {
 struct StateReport $C.mapname("state_report_t") {
   appliedLsn @0 :UInt64 $C.mapname("applied_lsn");   # standby's last applied WAL LSN
   configTag  @1 :UInt64 $C.mapname("config_tag");    # standby's current config version tag
+  term       @2 :UInt64 $C.mapname("term");          # standby's current raft term (bug-1421 term-discovery)
 }
 
 struct RequestVoteRequest $C.mapname("request_vote_req_t") {