feat(ws): RFC 6455 opening handshake — Conn::upgrade, 101 short-circuit (v0.4 chunk 1)

Design (as agreed in the v0.4 pass):
- Dep #3 spent: sha1_smol 1.0.1 (zero transitive deps) instead of a
  vendored SHA-1 — user call: no hand-rolled crypto. Consequence noted
  in ROADMAP: the v0.6 wire-format JSON question now costs dep #4.
- Base64 stays in-tree but ENCODE-ONLY (RFC 4648 vectors tested): it is
  an encoding, not crypto, and the decode direction (where parsing bugs
  live) is deliberately not implemented.
- src/ws/{mod,handshake}.rs: validate() implements §4.2.1 (GET, 1.1,
  Host, upgrade/connection token lists case-insensitively, key shape =
  base64 of exactly 16 bytes, version 13). Version mismatch -> 426 +
  sec-websocket-version: 13 (§4.2.2); everything else -> 400. Rejected
  upgrades stay plain HTTP with keep-alive intact (tested: 400 then 101
  on the same connection).
- Conn grows pub upgrade: Option<WsUpgrade> (opaque marker; chunk 3
  turns it into the duplex/handler handoff payload — shape deliberately
  uncommitted while the handler API is open). Conn::upgrade(self) is the
  commit point: 101 + accept key + marker + halt, or the rejection.
- conn_actor: upgrade marker + status 101 -> write head, leave the HTTP
  loop. Until chunk 3 that drops the fd (clients see 101 then EOF);
  status check is defensive against a post-handler plug clobbering 101.
- serialise_response: 1xx no longer get content-length injected
  (RFC 7230 §3.3.2). 204 left alone on purpose — separate conversation.

Accept-key path pinned to the §1.3 worked example end-to-end (unit +
wire). Suite: 34 unit + 33 integration + 2 doc.

src/conn_actor.rs

@@ -231,6 +231,20 @@ pub fn run_connection(
                 .put_body(RespBody::Empty);
         }
 
+        // ----- 3.5 WebSocket upgrade short-circuit. -----
+        // An accepted handshake (marker + 101) ends HTTP on this socket:
+        // write the 101 head and leave the request loop. Chunk 3 (duplex
+        // wiring) takes the socket over right here; until then leaving
+        // the loop closes it via OwnedFd::drop — the 101 is still the
+        // honest, testable wire artefact. The status check is defensive:
+        // a post-handler plug that clobbered the 101 forfeits the
+        // upgrade and falls through to plain HTTP.
+        if response_conn.upgrade.is_some() && response_conn.status == Some(101) {
+            let head = parser::serialise_response(&response_conn, true);
+            let _ = write_all(raw, &head, Instant::now() + limits.write_timeout);
+            return;
+        }
+
         // ----- 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