When the server advertises `accept-ranges: bytes` and a parquet exceeds
`AGNES_PULL_CHUNK_THRESHOLD_BYTES` (default 50 MB), `stream_download`
now splits the file into N parallel HTTP Range requests
(`AGNES_PULL_CHUNK_PARALLELISM`, default 4, capped 1..16) and
assembles the parts into the destination atomically.
Targets the per-flow-shaped network (corp VPN with per-TCP-connection
rate-limiting) where single-stream throughput is throttled but N parallel
streams over the same connection scale roughly linearly. Manifests with
1 large materialized parquet + N remote tables previously left the
existing across-files `AGNES_PULL_PARALLELISM=4` pool with 1 active
worker = single-stream throughput; this fixes that.
Falls back to single-stream when:
- HEAD doesn't advertise `accept-ranges: bytes`
- Server returns 200 instead of 206 to a Range probe
- File size below the threshold
Cleanup discipline: every part file removed before return (success or
failure); destination written via `<target>.tmp` and renamed atomically.
Per-chunk retry on transient network blips (bounded by AGNES_STREAM_RETRIES).
