feat: add streaming uploads, proxies, and HTTP/2

[Prodesire]

Req can now send file-backed and finite chunked request bodies, upload multipart files with form fields, and negotiate HTTP/2. Clients gain explicit proxy and connection-pool controls, while concurrent HTTP/2 streams share physical connections and stream cancellation preserves healthy peers.

Changes

• Add RequestBody and UploadFile with bounded file reads, multipart encoding, redirect replay, and one-shot consumption guards.
• Support HTTP and HTTPS proxies, CONNECT tunneling, environment proxy/CA settings, Limits, and pool timeouts.
• Add Client(http1=True, http2=False) protocol flags, ALPN negotiation, HTTP/1.1 fallback, and HTTP/2-only mode with capability checks.
• Correct HTTP/2 connection counting and upload rewind behavior, including refused-stream retries.
• Replace compatibility inventories with direct Mojo test discovery and organize cases under models, client, transports, and uploads; retain the existing third-party license notice.
• Add TLS HTTP/2 fixtures and CI test dependencies. Remove the version qualifier from both READMEs.
• Preserve Mojo string origins through native calls and construct safe empty proxy defaults, fixing Linux TLS and low-level stream failures.
• Complete peer GOAWAY shutdown in HTTP/2 fixtures and classify incomplete HTTP/2 responses as protocol errors across nghttp2 versions.

Validation

• make test: 384 passed, 0 failed, 0 skipped on local macOS, Ubuntu 24.04 ARM64, and the GitHub macOS 26.6.2 runner using system libcurl 8.7.1/nghttp2 1.68.1.
• make test-package: precompiled, isolated consumer and native-loading checks passed, including an unavailable HTTP/2 capability fixture.
• HTTP/2 suite against libcurl 8.10.1: 44 passed; HTTP/2-only negotiation repeated successfully 10 times.
• Latest PR CI at 33f4186: all six test/build/package jobs passed. Each test matrix job reports 384 passed, 0 failed, 0 skipped.
• git diff --check: passed.

HTTP/2-only mode requires libcurl 8.10 or later; negotiation with HTTP/1.1 fallback works with the system libcurl 8.7.1. The latest PR CI matrix covers Ubuntu 24.04 x86_64, Ubuntu 24.04 ARM64, and macOS 26.