refactor(redis): centralize miss and timestamp validation

[lan17]

Summary

Small, behavior-preserving cleanup following the reviews on #146. Rebuilt on main after #146 collapsed the miss union, which already absorbed most of the original scope (one miss shape, one constructor, CACHE_MISS_REASONS driving the type, isValidRedisTimestampMs owning the timestamp envelope). What remains:

• Move isCacheMissReason next to CACHE_MISS_REASONS in src/metrics.ts so the reason list has one owner; RedisCache imports it.
• Use isValidRedisTimestampMs in frameAge instead of a hand-written copy of the safe-integer envelope.
• Re-export CacheMissReason from dialcache/redis-protocol, where RedisReadMiss already lives; the packed consumer check asserts it is the root type.
• Comment the internal RedisCacheMissReason disposition type so it is not mistaken for the metric reason.
• Drop the README sentence duplicated between the network-shape and ACL paragraphs.
• Unit tests pin the guard against prototype keys and non-strings, and the timestamp predicate against coercion.

Scope and compatibility

No behavior change: validation sites and their order are unchanged, and the miss taxonomy, read-result shape, refill fencing, metrics, and Redis commands are untouched. The only public addition is the type-only re-export.

Validation

• pnpm typecheck
• 658 unit tests with coverage thresholds
• pnpm build and pnpm test:package