Add transport failure simulation coverage

[kacy]

Summary

This PR expands single-machine raft failure simulation coverage around transport-style faults and hardens one leader-side reply handling edge case that the new tests exposed.

What Changed

• Added raft unit coverage for:
  • stale failed append_entries_reply not regressing next_index
  • legitimate failed append_entries_reply backtracking only above the matched prefix
  • higher-term install_snapshot_reply forcing leader stepdown
• Hardened leader replication reply handling so a delayed failed append_entries_reply cannot backtrack below the follower's known matched prefix
• Extended the cluster simulation harness to model:
  • dropped requests
  • dropped replies
  • duplicate delivery
• Added simulation coverage for:
  • dropped append_entries_reply recovering on heartbeat retry without duplicating log entries
  • duplicate append_entries delivery being idempotent
  • dropped install_snapshot_reply recovering on retry without regressing follower state
  • duplicate install_snapshot delivery being idempotent
  • dropped request_vote_reply messages recovering on the next election round
  • 5-node quorum commit under mixed dropped replies, duplicate delivery, and lagging follower repair
  • snapshot retry after follower restart resuming replication cleanly

Why

The earlier simulation work covered dropped delivery and restart continuity, but it still left important transport-fault classes under-tested:

• delayed negative replies arriving after newer successful replication
• duplicated RPC delivery
• lost vote replies and lost snapshot replies
• mixed-fault conditions where quorum should still commit while laggards recover later
• snapshot retry behavior across follower restarts

These tests improve confidence in raft behavior on a single machine without needing real distributed fault injection.

Validation

All test runs were executed serially with YOQ_SKIP_SLOW_TESTS=1.

• zig build test -Doptimize=ReleaseSafe -Dtest-filter='append_entries_reply'
• zig build test -Doptimize=ReleaseSafe -Dtest-filter='leader steps down on higher term in install_snapshot_reply'
• zig build test-sim -Doptimize=ReleaseSafe -Dtest-filter='dropped append_entries replies recover on heartbeat retry without duplicating log entries'
• zig build test-sim -Doptimize=ReleaseSafe -Dtest-filter='duplicate append_entries delivery is idempotent'
• zig build test-sim -Doptimize=ReleaseSafe -Dtest-filter='dropped install_snapshot reply recovers on retry without regressing follower state'
• zig build test-sim -Doptimize=ReleaseSafe -Dtest-filter='duplicate install_snapshot delivery is idempotent'
• zig build test-sim -Doptimize=ReleaseSafe -Dtest-filter='dropped request_vote replies recover on the next election round'
• zig build test-sim -Doptimize=ReleaseSafe -Dtest-filter='5-node mixed transport faults still commit with quorum and repair laggards later'
• zig build test-sim -Doptimize=ReleaseSafe -Dtest-filter='snapshot retry after follower restart resumes replication cleanly'