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>

desktop/src-tauri/src/lib.rs

@@ -821,6 +821,15 @@ async fn register_signal(
                     tracing::warn!("signal recv returned None — peer closed");
                     break;
                 }
+                Err(wzp_proto::TransportError::Deserialize(e)) => {
+                    // Forward-compat: the relay sent us a
+                    // SignalMessage variant we don't know yet
+                    // (older client against a newer relay).
+                    // Log and keep the signal connection alive —
+                    // otherwise direct-call registration would
+                    // silently die on any protocol bump.
+                    tracing::warn!(error = %e, "signal recv: unknown variant, continuing");
+                }
                 Err(e) => {
                     tracing::warn!(error = %e, "signal recv error — breaking loop");
                     break;