Prototype elastic reclaim and add reclaim benchmark

[caomengxuan666]

Summary

• Add opt-in elastic reclaim prototype while keeping stable and idle_reclaim behavior unchanged.
• Add benchmark coverage for memory-cycle behavior and dedicated reclaim-only cost.
• Document the elastic reclaim design, current limitations, and benchmark gates.

Validation

• Built and linked mpsc_bench_reclaim in Release.
• Ran mpsc_bench_reclaim across reserve sizes 1, 4, 16, 64, and 256 chunks.
• Existing tests and broader throughput/latency/soak coverage should still be reviewed before marking this ready to merge.

Notes

• This is intentionally opt-in and experimental.
• The default queue behavior remains unchanged.
• reclaim_free_pages() is currently a quiescent reclaim hook, not a fully concurrent online shrinker.