fix: raft replication match_index regression and stale truncation term

[kacy]

Summary

• enforce match_index monotonicity in handleAppendEntriesReply — stale/reordered replies can no longer regress a peer's tracked replication state
• enforce match_index monotonicity in handleInstallSnapshotReply — same protection for snapshot RPCs
• fix stale existing_term usage after log truncation in handleAppendEntries — after truncating conflicting entries, the code was using the pre-truncation term value for the append decision, potentially skipping entries that should be appended

3 regression tests covering: out-of-order reply handling, truncation+append correctness, and snapshot reply monotonicity.