feat(npm): packument GET cache-hit serve (S11 Step 11 part 2/4)

📦 Summary

Part 2 of 4 of the npm hosted packument GET handler (S11 Step 11). Builds on part 1 (core serve) to add the cache-hit path:

• npm_metadata_files cache lookup in ServeHTTP, dispatching to a cache-hit branch when a fresh row exists
• serveCacheHit streaming the pre-rendered blob via BlobStore.OpenBlob
• Strong ETag derived from the cache row's stored blob_sha256
• sha256SumFromBytes helper

Constructor grows to take the cache and blobs (BlobStore) dependencies.

🧪 Tests

Unit: inline-build performs no blob open, cache-hit serves via OpenBlob, no synthesized latest. Integration: inline-build after publish.

🗂️ Stack

Part 2/4, targets 10io/npm-hosted-step11-part1of4 (part 1/4). Part 3 stacks on this.

Related to #129 (closed)

🔗 Stacked MRs (review/merge bottom-up)

• feat(npm): packument GET core serve (S11 Step 1... (!630 - merged) • David Fernandez • 19.2
• feat(npm): packument GET cache-hit serve (S11 S... (!631 - merged) • David Fernandez • 19.2 👈
• feat(npm): packument GET conditional GET + cach... (!632 - merged) • David Fernandez • 19.2
• feat(npm): packument GET inline-build singlefli... (!633 - merged) • David Fernandez • 19.2