perf: use ArcSwap in EphemeralFile to eliminate read stalls

Replaces the RwLock<BufferedWriter> with a lock-free ArcSwap<FrozenState> pattern.

Problem:
Previously, flushing the EphemeralFile to disk held a write lock on the struct.
During slow disk IO (10ms - 50ms+), this blocked all concurrent readers (GetPage),
causing latency spikes and throughput collapse.

Solution:
1. Decouple the 'Frozen' flush buffer from the 'Mutable' tail.
2. Use `arc_swap::ArcSwap` to publish the frozen buffer atomically. Readers access it wait-free.
3. Add `disk_committed_offset` to distinguish between data in the buffer vs. disk, preventing race conditions.
4. Add `writer_lock` to serialize concurrent writers (WAL append-only integrity) without blocking readers.

Benchmarks (Local NVMe + Synthetic 50ms Latency):
- Read Throughput: ~7.4x improvement (43k -> 323k ops/sec).
- Write Throughput: ~1600-2300 MB/s (Comparable to baseline).
- Fixes #12172

Author: armstrong0704