Built meta.idx and meta.dat. Binary index that sits right next to the artifact files. Key goes in, offset comes out, metadata comes back. Under 5ms for 500 lookups.
3 architectures in 2 weeks to get here. Worker-local lookup, artifact server API, now this. The first two taught me what the access pattern actually is -- lots of point reads by key, right after a search, latency-sensitive. Once you know that, the answer is obvious. Sorted index, binary search, memory-mapped data file.
It's ugly. The index is a flat sorted array of fixed-width key hashes with offsets into the data file. No compression, no cleverness. But it works and it's fast and I'm done touching metadata lookups for a while.