stream: set_negotiated_key, one place that maps the picked version to a key

Every handshake picked a version and then repeated the same choice: at
least 1 splits the key, 0 sets it as before. Stream now makes that choice
from the transcript it is already given, so callers make one call and a
later version is handled here rather than at each of them. Version 0
still goes through set_key.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019aokqJuhjvB3kijXtAg5Ns

src/stream.rs

@@ -80,15 +80,20 @@ impl Stream {
         }
     }
 
-    /// Per-direction keys bound to the handshake; see `tcp::KX_VERSION_LATEST`. `is_initiator`
-    /// is the side that sent the sealed key.
+    /// The stream key for the key exchange version in `t.picked`: version 0 is `set_key`, and
+    /// later versions derive one key per direction from the transcript. `is_initiator` is the
+    /// side that sent the sealed key.
     #[inline]
-    pub fn set_key_split(
+    pub fn set_negotiated_key(
         &mut self,
         key: Key,
         is_initiator: bool,
         t: &tcp::KxTranscript,
     ) -> ResultType<()> {
+        if t.picked == 0 {
+            self.set_key(key);
+            return Ok(());
+        }
         match self {
             #[cfg(feature = "webrtc")]
             Stream::WebRTC(s) => {