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-relay/src/main.rs

@@ -1307,6 +1307,16 @@ async fn main() -> anyhow::Result<()> {
                             info!(%addr, "signal connection closed");
                             break;
                         }
+                        Err(wzp_proto::TransportError::Deserialize(e)) => {
+                            // Forward-compat: the peer sent a
+                            // SignalMessage variant we don't know
+                            // (newer client, newer federation peer).
+                            // Log and continue — tearing down the
+                            // connection on unknown variants would
+                            // silently kill interop across minor
+                            // protocol version bumps.
+                            warn!(%addr, "signal deserialize (unknown variant?), continuing: {e}");
+                        }
                         Err(e) => {
                             warn!(%addr, "signal recv error: {e}");
                             break;