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/ws/mod.rs

@@ -0,0 +1,26 @@
+//! WebSocket support (RFC 6455).
+//!
+//! v0.4 chunk 1: the upgrade handshake. A handler that wants to speak
+//! WebSocket calls [`Conn::upgrade`](crate::Conn::upgrade); on a valid
+//! handshake the connection actor writes the `101 Switching Protocols`
+//! response and leaves the HTTP request loop. (Until the duplex wiring
+//! lands — chunk 3 — leaving the loop closes the socket; the 101 itself
+//! is correct and tested.)
+//!
+//! Dependency note: SHA-1 comes from `sha1_smol` (zero transitive deps),
+//! spending urus dependency slot #3 — agreed in the v0.4 design pass over
+//! vendoring. Base64 here is the 20-byte *encode* direction only and is
+//! not cryptographic, so that stays in-tree (`handshake::b64`).
+
+pub mod handshake;
+
+pub use handshake::Rejection;
+
+/// Marker carried on a [`Conn`](crate::Conn) whose handler accepted a
+/// WebSocket upgrade. Opaque: chunk 3 grows this into the handler/duplex
+/// handoff payload, so nothing outside the crate should depend on its
+/// shape.
+#[derive(Debug)]
+pub struct WsUpgrade {
+    pub(crate) _priv: (),
+}