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).

ROADMAP.md

@@ -229,21 +229,19 @@ PubSub underneath.
 
 ## Known bugs
 
-- **HTTP/1.0 keep-alive: server honors but never advertises** (found
-  2026-06-12 via `ab -k`). A 1.0 request with `Connection: Keep-Alive`
-  IS kept alive (second pipelined request on the same socket gets
-  answered), but the response carries no `Connection: Keep-Alive`
-  header. Per 1.0 convention the client may only reuse when the server
-  explicitly opts in; absent the header, clients assume the server will
-  close and wait for EOF — which never comes. `ab -k` therefore
-  deadlocks until its poll timeout (`apr_pollset_poll: timeout 70007`)
-  on the FIRST response; no keep-alive bench numbers obtainable.
-  Fix direction: when the conn stays alive on a 1.0 exchange, echo
-  `Connection: Keep-Alive` in serialise_response — or don't keep 1.0
-  alive at all. Decide scope (error responses, HEAD) with user first.
-  Baseline for reference: GET /users (crud example, release, loopback,
-  stdout logging silenced), no keep-alive, c=50: ~18.9k req/s, 0
-  failed, p50 3ms / p99 5ms. HTTP/1.1 path responds normally.
+- ~~**HTTP/1.0 keep-alive: server honors but never advertises**~~ FIXED
+  (2026-06-12, same session it was found). As-landed decisions:
+  serialise_response now echoes `connection: keep-alive` on a kept-alive
+  1.0 response (1.1 keep-alive stays implicit); error statuses (>=400)
+  on 1.0 force close — no keep-alive advertised on a 4xx/5xx where reuse
+  is opt-in (conn_actor, alongside the existing 1.0-stream force-close);
+  HEAD needs nothing special (header echo only, framing untouched);
+  parse-error responses already carried `connection: close`. Validated
+  against the original repro: `ab -k -n 5000 -c 50` on GET /users now
+  reports 5000/5000 keep-alive requests, 0 failed, ~63k req/s
+  (vs ~18.9k no-keep-alive baseline, same box). Watch out benching:
+  ab counts a response toward "Non-2xx" silently — a 404'd route still
+  shows "Failed requests: 0" with great rps; check the route first.
 
 ## Later / icebox