core: add webrtc-rs (validated host + iOS); SSE forwards clipboard only

Add the webrtc dependency that the in-core RTC transport will use. Verified
it cross-compiles to aarch64-apple-ios, confirming RTC can live in the shared
core and reach every native platform from one implementation (the browser
keeps native RTCPeerConnection and interops over the wire).

Also fix a latent bug: the SSE transport forwarded every message type to the
sink, so a peer's presence/signal chirps would leak into the clipboard feed.
Now SSE forwards only clipboard; presence/signal belong to the RTC layer.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/Cargo.toml

@@ -22,3 +22,4 @@ serde_json = "1"
 reqwest = { version = "0.12", default-features = false, features = ["rustls-tls", "stream", "json"] }
 tokio = { version = "1", features = ["rt-multi-thread", "time", "sync", "macros"] }
 futures-util = "0.3"
+webrtc = "0.17.1"

core/src/lib.rs

@@ -275,7 +275,9 @@ impl SseTransport {
                 if line.is_empty() {
                     if !data.is_empty() {
                         if let Ok(m) = serde_json::from_slice::<Message>(&data) {
-                            if m.from != self.cfg.from {
+                            // Clipboard only — presence/signal chirps on the bus
+                            // belong to the RTC layer, not the feed.
+                            if m.from != self.cfg.from && (m.kind.is_empty() || m.kind == "clipboard") {
                                 sink(m); // engine de-dups + forwards to handler
                             }
                         }