feat(store): expose cache hit ratio metric

[loglapa]

Closes #233

Overview

• expose hdr_store_cache_hit_ratio_gauge as a dimensionless 0..1 ratio
• count only reads actually served by the in-memory cache or datastore, excluding pending writes
• add focused miss-then-hit coverage

Validation

• go test ./store -count=1
• go test -race ./store -run TestStore_CacheHitRatio -count=1
• golangci-lint run ./store

The full go test ./... run also reaches an unrelated existing panic in TestExchangeServer_partialRangeNotExpanded; the changed store package and focused race test pass.