* feat(storage): expose efficient seek for local files
Open file-backed artifacts with os.Open so local readers support
efficient seeking. Leave cloud storage readers unchanged.
Keep the legacy sidecar mapping test focused on fileblob's reader:
Blob.Open now reads local files directly and no longer consults the
sidecar. Continue checking sidecar cleanup and post-store reads.
* feat(handler): preserve efficient seek through integrity checks
Keep the cache result's single Reader field and preserve io.Seeker
when the underlying storage reader supports efficient seeking.
Sequential reads continue through whole-object integrity verification.
A successful seek switches subsequent reads to the source for range
responses, where full-object verification is not possible.
Test both modes and verify the source is closed once.
* feat(handler): add request-aware byte range responses
Add a request-aware artifact serving helper for bounded, open-ended,
and suffix byte ranges, including If-Range handling, 206 responses,
and 416 responses for valid unsatisfiable ranges.
Ignore malformed and multi-range requests by serving the full artifact.
Advertise byte-range support only for seekable readers, preserving HEAD,
redirect, and non-seekable behavior. Clarify that net/http recovers
ErrAbortHandler per request and keeps the server running.
* feat(handler): enable byte ranges across artifact downloads
Forward each artifact request to the shared range-aware response helper,
including cached generic release assets and OCI blobs. This lets the
handler honor ranges only when its reader supports efficient seeking,
without changing direct-storage redirect behavior.
Add warm-cache endpoint tests that assert partial response headers and
bytes without an upstream refetch. Document supported range behavior,
unsupported reader fallback, and the integrity limitation of partial reads.
Every built-in ecosystem upstream can now be set via the upstream
config block or PROXY_UPSTREAM_* env vars, with allow_private_hosts
and allow_loopback controlling access to non-public addresses.
