feat(network): poll-driven HTTP server and client over TcpNetwork

[louisponet]

Summary

Adds flux_network::http::HttpNetwork — a poll-driven HTTP/1.x layer on top of TcpNetwork. One instance runs server and client in a single event loop: it can listen for inbound requests and maintain persistent outbound endpoints, delivering parsed requests/responses as borrowed events through poll_with.

HTTP layer (http.rs, new)

• Server: listen + HttpEvent::Request; responses via respond(token, status, headers, body), deferrable to a later poll (one pending request per connection, pipelined requests stay buffered behind it). Handles Expect: 100-continue, HEAD semantics, HTTP/1.0/1.1 keep-alive vs close, and protocol errors (400/413/431/501) before user code sees anything.
• Client: connect + request(token, method, path, headers, body) (one in flight per endpoint); responses support Content-Length, chunked, and EOF-delimited bodies, with interim responses skipped.
• Configurable head/body/header-count limits and idle timeout for accepted connections. Routing is intentionally left to the caller (match on request.method / request.path).
• Head parsing uses httparse (new workspace dep); body/chunked handling is in-crate.

TcpNetwork changes (network.rs)

• Per-group Framing: existing LengthPrefixed (default, unchanged) or new Raw passthrough. Raw groups cap read chunks at max_frame_size, stamp events with local receive time, and skip latency telemetry (no wire timestamps).
• disconnect_when_drained(token): close after queued bytes flush (used for Connection: close responses); sends to a draining token are rejected.
• ByteQueue remainder path generalized to an optional header prefix to serve both framings.

Testing

• just fmt / just clippy clean (zero warnings).
• New tests: 872-line HTTP integration suite (tests/http.rs), raw-framing suite (tests/tcp_network_raw.rs), and byte-queue unit tests.
• cargo test --workspace --all-features --locked passes. Note: in repeated local runs the pre-existing tcp_multi_client_backpressure test flaked once under heavy machine load (fixed 5s deadline pushing 8MB through 1KB socket buffers); it exercises TcpConnector (connector.rs/stream.rs), which this PR does not touch, and passes on retry.