Pool the httpx.Client used by `stream_download` so N parquet downloads
share a single TLS handshake instead of one handshake each. With the
optional `h2` package installed, HTTP/2 multiplexing further lets all
chunk Range requests share a single TCP connection — synergizes with
the range-chunked download path added in the previous commit.
The shared client is created lazily on first stream-download call, kept
alive for the duration of the process via a module-level slot, and
closed at exit via `atexit.register`. Construction wraps in a
try/except: when `h2` is unavailable (slim install), httpx raises
ImportError on `http2=True` and we transparently fall back to an
HTTP/1.1 client — pooling alone still amortizes TLS handshakes.
`agnes pull` must never crash on a missing optional package, so the
fallback path is non-negotiable. `h2>=4.1.0` is added to the core
dependency set; downstream slim installs that drop it lose the HTTP/2
benefit but keep correctness.
