making append log entries atomic in nature

[codecasualty]

What this does

Three main changes:

1. Fixed follower crash after failover

• A restarted node could crash because commitIndex temporarily pointed beyond the in-memory log while AppendEntries was updating it.
• Fixed by writing to the WAL first, then atomically truncating and appending to the in-memory log under one lock.
• Added WAL TRUNCATE records and prevented failed WAL writes from leaving a partially modified log.
• Verified with unit tests, integration/durability tests, and two live 3-node chaos runs.

2. Disabled benchmark logging by default

• METRICS logging was accidentally enabled at INFO, generating heavy disk I/O and competing with WAL fsyncs.
• Now disabled by default; enable it explicitly for benchmarks.

3. Notes

• Follower still truncates beyond prevLogIndex even without a conflict; safe but inefficient.
• The apply loop should additionally enforce lastApplied <= log.lastIndex().