fix(signal): forward-compat — log+continue on unknown SignalMessage variants

Both sides of the signal channel previously broke their recv loop
on any deserialize error, which meant adding a new variant in one
build silently killed signal connections from peers running an
older build. This bit us during Phase 1 testing: a new client
sending SignalMessage::Reflect to a pre-Phase-1 relay caused the
relay to drop the whole signal connection, which looked like
"Error: not registered" on the next place_call.

Fix:
- New TransportError::Deserialize(String) variant in wzp-proto
  carries serde errors as a distinct category.
- wzp-transport/reliable.rs::recv_signal returns Deserialize on
  serde_json::from_slice failures (was wrapped in Internal).
- wzp-relay/main.rs signal loop matches on Deserialize → warn +
  continue (instead of break).
- desktop/src-tauri/lib.rs recv loop does the same.

Other TransportError variants (ConnectionLost, Io, Internal) still
break the loop — only pure parse failures are recoverable.

This means future SignalMessage variant additions are backward-
compat by construction: older peers will see "unknown variant,
continuing" in their logs while newer peers can keep evolving the
protocol.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-transport/src/reliable.rs

@@ -53,6 +53,13 @@ pub async fn recv_signal(recv: &mut quinn::RecvStream) -> Result<SignalMessage,
         .await
         .map_err(|e| TransportError::Internal(format!("stream read payload error: {e}")))?;
 
-    serde_json::from_slice(&payload)
-        .map_err(|e| TransportError::Internal(format!("signal deserialize error: {e}")))
+    serde_json::from_slice(&payload).map_err(|e| {
+        // Distinguish serde failures from transport failures so the
+        // caller (relay main loop, client recv loop) can continue on
+        // unknown-variant / parse errors instead of tearing down the
+        // whole signal connection. Forward-compat: adding a new
+        // `SignalMessage` variant in one side must not break the
+        // other side's signal connection.
+        TransportError::Deserialize(format!("{e}"))
+    })
 }