Add durability callbacks and wait APIs for sync writes

Normalized profile

What this task asks for

Add batch durability callbacks, durability wait methods, notifications, and metrics for sync write commits. BatchDurableInfo carries JobID int, SeqNum base.SeqNum, Err error, ApplyDuration time.Duration, SyncDuration time.Duration, CorrelationID uint64 (from WriteOptions.CommitCorrelationID uint64), BatchSize int (encoded batch size in bytes), and KeyCount uint32. Non-sync commits and DisableWAL must never trigger it. Jobs outside a bounded retention window get a distinguishable "expired" error (message must contain "expired"). Never-seen and zero IDs get an "unknown" error (message must contain "unknown"). When DisableWAL is true, wait APIs and DurabilityNotify return nil immediately. Expose Metrics.DurableCommitCount uint64 and Metrics.DurableCommitDuration time.Duration (cumulative WAL sync phase time, not total commit time), accumulated only when BatchDurable is configured.