test(raft-ha-core-safety): end-to-end (term,payload) fidelity across replication + catch-up (task 6.1)

Integration test proves the guarantee tasks 2.3/2.4 hardened in
isolation (synthetic fault injection on the streamer) also holds
end-to-end over two real persist_t/WAL instances through the real
production ingest seam (ha_apply_wal_catchup_entry). Covers live
in-term replication and cross-term catch-up: no term==0 sourced for a
live entry, no non-empty entry ever lands as PLEN=0, standby persists
the entry's stored term (not the AE header's leader-current-term,
the bug-1067 split), and walcompare reports a clean committed prefix.

test_ha_core_safety 26/0, no regression, no ASAN reports.

.kiro/specs/raft-ha-core-safety/tasks.md

@@ -181,7 +181,7 @@
   - _Depends: 5.9_
 
 - [ ] 6. Integration: single-seam invariant end-to-end
-- [ ] 6.1 Verify leader→standby replication carries (term, payload) faithfully end-to-end
+- [x] 6.1 Verify leader→standby replication carries (term, payload) faithfully end-to-end
   - Integration check that the live leader replication AND the cross-term catch-up streaming path transmit each entry's full `(term, payload)` to the receiver: no `term==0` on the wire from a live leader (defensive), and — the real concern — no non-empty entry ever arrives with `PLEN=0`; the walAck catch-up continuation feeds only the seam ingest path
   - Observable completion: an end-to-end test (or instrumented soak round) shows leader and standby hold identical `(term, payload-crc)` for every replicated and cross-term-caught-up LSN; `walcompare --report` clean across members
   - _Requirements: 2.1, 2.3, 3.2_