fix: decode Transfer-Encoding: chunked request bodies

[maddie]

Problem

The BodyType::Chunked branch (src/http/request.rs) read the raw stream in fixed 1024-byte read_exact blocks and never chunk-decoded it. With a finite payload the client keeps the connection open (keep-alive, no half-close), the final block never fills, and the server blocks forever instead of answering — hanging any chunked uploader after its first payload.

Concrete case: librespeed/speedtest-cli#122 — librespeed-cli uploads at 0.55 Mbps against librespeed-rs (one 1 MiB payload / 15 s duration). Same stall reproduces with curl -H "Transfer-Encoding: chunked" --data-binary @file.

See librespeed/speedtest-rust#49 for the full analysis (server log: CHUNKED: read 1048837 raw bytes, then EOF/blocked — 1024×1024 payload + 261 bytes of chunk framing is not a multiple of the 1024-byte read block).

Fix

Decode the chunked framing per RFC 9112 §7.1: read the chunk-size line (extensions after ; ignored), the chunk data, and the trailing CRLF per chunk; stop at the 0-size last chunk and consume the trailer section up to the blank line.

Verification

• cargo build --release passes.
• Against a running server: curl -H "Transfer-Encoding: chunked" --data-binary @1MiB → HTTP 200, 0.001s (previously hung); Content-Length uploads still work.
• End-to-end with the fixed client (fix: set upload Content-Length to the payload size, not the KiB option speedtest-cli#144): upload 23192 Mbps on loopback (was 0.55–1.61 Mbps).

Closes #49