Fixes#78
## Why
`serveFromStore` first called `store.Download` with the bare content hash as the S3 key, which never matches real object keys (`blobs/sha256/<hash>`). Every cached blob serve therefore paid an extra guaranteed-404 round-trip before retrying with the correct `BlobKey`.
## Changes
- Remove the dead first `Download` attempt; go straight to the `BlobKey` lookup, then fall back to the index key.
## Validation
- `make e2e` passes (proxy cache-hit paths exercised end-to-end).
Reviewed-on: #82
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>
unkinben
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