Download Px 0.13.1

Improvements
- --authretry now defaults to 600 seconds so a blocked upstream proxy recovers without a restart; use --authretry=0 for the old behaviour.
- Requires pymcurl 8.23.0.3 or newer (fixes #268).
- Release binaries and the Docker image now use CPython 3.14 (px#280).
Bug fixes
- px --test paces body-echo checks and reports rate-limited targets separately from Px failures.
- Parallel uploads no longer fail when many new connections resolve the same upstream proxy concurrently.
- Upstream proxy DNS seeding now expires normally and retains IPv4/IPv6 failover addresses.
- Clients using Expect: 100-continue now receive one 100 Continue per upload instead of a duplicate relayed from the upstream proxy.
- Plain-HTTP uploads through authenticated proxies no longer fail or buffer request bodies (#199).
- A slow or stalled upload no longer delays other requests handled by the same worker.
- A client Connection: close (and other hop-by-hop headers) is no longer forwarded upstream, which broke NTLM through proxies that honour it.
- NTLM/Negotiate upstream proxies no longer block after interrupted CONNECT handshakes (px#268).
- A request body sent late after a client-authentication 407 to Expect: 100-continue is no longer parsed as the next request.
- Failed downstream client authentication now returns a fresh 407 instead of a 401, and NEGOTIATE/NTLM restart correctly after a failed exchange.
- px --test/--test-auth now fail requests ending with 401/407 (px#282).
- --test-auth with Negotiate and one explicit proxy now uses the upstream proxy hostname.
- Downstream NTLM/Digest/Basic authentication no longer stalls on repeated password lookups.
- px --test output is retained when piped.
Internal
- The corporate-proxy Docker scenario suite runs in CI and ./build.sh release_check.
- Release publishing is split between build.yml (GitHub release/PyPI) and publish.yml (Docker/winget).