feat(conn): streaming response bodies — RespBody::Stream + chunked TE (v0.3 chunk 1)

Design decisions (per handoff Q1 user answer + Q3 proposal):

- Producer shape is PULL: the handler returns a smarm Receiver<Vec<u8>>
  (RespBody::Stream / StreamBody, From<Receiver<Vec<u8>>> for ergonomics);
  the producer is an actor the handler spawned. The conn actor pumps in
  pump_stream(): it keeps sole ownership of the socket and of write
  deadlines, and the recv() park between chunks is stoppable by the
  draining registry's request_stop (stop sentinel unwinds out of
  park_current; fd + registry guards clean up) — so an infinite stream is
  force-stoppable at the drain deadline like any in-flight request, with
  no polling. End of stream = every Sender dropped = terminating 0-chunk.
  Producer contract: a send() error means the conn died; exit.

- Framing: HTTP/1.1 gets transfer-encoding: chunked and the connection
  stays reusable after the terminator (keep-alive after chunked). HTTP/1.0
  has no chunked TE: bytes go raw, keep-alive is forced off, EOF delimits.
  User-set content-length/transfer-encoding headers are DROPPED for stream
  bodies — we own the framing, and CL+chunked is a smuggling vector.
  Empty producer chunks are skipped (a 0-length chunk would terminate the
  framing early).

- request_timeout stays READ-phase only (unchanged). NEW Config/ConnLimits
  field write_timeout (default 30s) gives every response write a per-write
  budget: the fixed head+body write, each streamed chunk, and the error
  paths (413/100-continue/4xx) all go through the now deadline-bounded
  write_all (wait_writable_timeout, mirroring read_some). Each chunk gets
  a FRESH budget — streams may outlive any whole-response clock; a single
  stalled write may not (write-side slowloris). Naming/default were
  flagged as a user call in the handoff: veto here if write_timeout(30s)
  isn't it.

- RespBody loses derive(Clone) (Receiver isn't Clone; Clone was unused)
  and gets a manual Debug.

Tests: 3 serialiser unit tests (chunked head shape, user-framing-header
stripping, 1.0 fallback) + 5 integration (chunked round-trip with decoder,
keep-alive after chunked, 1.0 EOF-delimited, shutdown force-stops an
infinite stream at drain deadline, stalled reader killed at write_timeout
with producer observing the closed channel).

src/lib.rs

@@ -28,7 +28,7 @@ pub mod conn_registry;
 pub mod serve;
 
 // Re-exports — what most users want at the crate root.
-pub use conn::{Assigns, Body, Conn, HeaderMap, HttpVersion, Method, Params, RespBody};
+pub use conn::{Assigns, Body, Conn, HeaderMap, HttpVersion, Method, Params, RespBody, StreamBody};
 pub use plug::{Next, Pipeline, Plug};
 pub use router::Router;
 pub use serve::{