perf(kv): reduce KV round-trips on cache hit via local tag cache

[Divkix]

Problem

When KVCacheHandler.get() gets a cache hit, it validates every tag by issuing one kv.get() per tag in parallel (Promise.all). For entries with 20 tags, that's 20 KV round-trips per cache hit. Same pattern in revalidateTag() — N parallel PUTs.

Solution

Add a local in-memory Map<string, { timestamp, fetchedAt }> with a 5-second TTL that caches tag invalidation timestamps. Within the TTL window, tag checks are served from memory with zero I/O. After TTL expiry, the next request re-fetches from KV.

Key behaviors:

• revalidateTag() updates the local cache immediately so invalidations are reflected without waiting for TTL expiry
• resetRequestCache() clears the local cache for per-request isolation
• NaN tag timestamps are cached and correctly treated as invalidation
• Only uncached/expired tags trigger KV reads (partial cache hits work)

Implementation

PR: #433