Defer cache index touch writes

[jerenkrantz]

Summary

• admit only expired LRU entries to a bounded, deduplicating touch worker
• move LRU bookkeeping off cache-hit paths and batch deferred timestamp updates
• avoid syncing eviction hints per read; refcount mutations retain their synchronous durability
• cover freshness filtering, coalescing, and non-blocking local reads

Verification

• go build ./...
• go vet ./...
• go test -race ./...