fix(http): advertise keep-alive on HTTP/1.0 responses we keep open

1.0 defaults to close: honoring 'Connection: Keep-Alive' without echoing
it back left spec-following clients waiting for an EOF that never came
(ab -k deadlocked to its poll timeout on response 1).

As agreed:
- serialise_response echoes 'connection: keep-alive' when keep_alive
  is on AND version is 1.0. 1.1 keep-alive stays implicit.
- Error statuses (>=400) on 1.0 force keep_alive off in the conn actor
  (joins the existing 1.0-stream force-close): we don't advertise reuse
  on a 4xx/5xx to a protocol generation where reuse is opt-in.
- HEAD needs nothing: the echo is framing-neutral.
- Parse-error responses already carried 'connection: close'.

Tests: unit pair (1.0 echo / 1.1 implicit) + integration pair (socket
actually reused on 1.0; 404 forces close + EOF). Validated against the
original repro: ab -k -n 5000 -c 50 GET /users -> 5000/5000 keep-alive,
0 failed, ~63k rps (~18.9k without keep-alive).

src/conn_actor.rs

@@ -248,10 +248,13 @@ pub fn run_connection(
         // ----- 4. Write the response. -----
         // A Stream body on HTTP/1.0 has no chunked framing: the body is
         // delimited by EOF, so keep-alive is forced off for this response
-        // (and `connection: close` goes on the wire).
+        // (and `connection: close` goes on the wire). Error statuses on
+        // 1.0 also force close: we don't advertise keep-alive on a
+        // 4xx/5xx to a protocol generation where reuse is opt-in.
         let is_stream = matches!(response_conn.resp_body, RespBody::Stream(_));
+        let is_error  = response_conn.status.unwrap_or(200) >= 400;
         let keep_alive = keep_alive
-            && !(is_stream && version == HttpVersion::Http10);
+            && !(version == HttpVersion::Http10 && (is_stream || is_error));
 
         let head_bytes = parser::serialise_response(&response_conn, keep_alive);
         if write_all(raw, &head_bytes, Instant::now() + limits.write_timeout).is_err() {