feat(raft-ha-core-safety): establish snapshot (term,lsn) boundary on install (task 3.2)

Adds checkpointTerm (@6, append-only) to the SnapshotTransfer wire
message. The leader stamps it via persist_entry_term(p, checkpoint_lsn)
when sending a snapshot; the standby reconciles it into its own
per-LSN term index (persist_reconcile_checkpoint_term) right after
persist_load_snapshot succeeds, before applied_lsn advances. This
authoritatively overwrites any stale local term a divergent
pre-snapshot entry left at the checkpoint LSN, closing the gap task
3.1's RED test proved (Expected 7 Was 3 -> now passes). Transport
stays persist_t-free; the reconcile lives in persist.c per the
existing dependency direction. Decode is auto-generated via capnp
mapname; only the hand-rolled encode path (request.c/typemap.h, needed
for the raw snapshot payload) required a matching manual update.

test_ha_core_safety 25/0, test_config_repl 66/0, no ASAN reports.

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

@@ -73,7 +73,7 @@
   - _Requirements: 3.1, 5.1_
   - _Boundary: Snapshot Boundary Reconcile_
 
-- [ ] 3.2 Establish the snapshot `(term, lsn)` boundary
+- [x] 3.2 Establish the snapshot `(term, lsn)` boundary
   - Make snapshot install establish a verifiable `(term, lsn)` at the checkpoint so post-install AE intake reconciles correctly and no committed entry below the checkpoint can diverge
   - Observable completion: 3.1 passes; a member that installed a snapshot matches the leader's `(term, payload-crc)` at and below the checkpoint boundary
   - _Requirements: 3.1, 5.1_