[V2] Double-buffering for concurrent flush

[bravo1goingdark]

Summary

Implement double-buffering to allow concurrent memtable flush while writers continue to a new active memtable.

Motivation

Currently, seal() blocks all writes during the flush phase. Double-buffering would allow writers to continue to a new arena while the frozen memtable is being iterated/flushed.

Implementation Notes (from design doc)

If you wanted to support concurrent memtable flush while writers continue (double-buffering), you would need:

1. Two arenas: active (writers) and frozen (being flushed)
2. Writers atomically switch to the new arena
3. The frozen arena is immutable — safe to iterate without synchronization

From design doc

See LOCKFREE_SKIPLIST_DESIGN.md lines 592-595

Priority: low