perf(mst2): share concurrent cold chunk projections (#65)

Deduplicate concurrent same-digest cold projection loads, retain verified results for active participants after cache eviction, and preserve full SHA/map/slice validation. Bound registry keys and clean up exact last-owner gates without dropping large results under the global lock. Add ten failure/cancellation/eviction/capacity regressions. Native and performance evidence remain separate.

Author: Ivanbeethoven