feat: bidirectional signal streaming sync

- _signalRegistry: maps signal names → signal objects
- _registerSignal: called after each DS.signal() declaration
- _applyRemoteDiff: JSON diff → update local signals + flush
- _initStreamReceiver: parallel receiver WS for incoming diffs
- Echo loop guard: _applyingRemoteDiff prevents re-broadcasting
- 0x31 handler in _handleRemoteInput for signal diff frames

Changes sync both directions: laptop → phone, phone → laptop.

compiler/ds-codegen/src/js_emitter.rs

@@ -104,6 +104,8 @@ impl JsEmitter {
                         }
                     };
                     self.emit_line(&format!("const {} = DS.signal({});", node.name, init));
+                    // Register signal for bidirectional streaming sync
+                    self.emit_line(&format!("DS._registerSignal(\"{}\", {});", node.name, node.name));
                 }
                 SignalKind::Derived => {
                     // Find the let declaration to get the expression
@@ -1814,6 +1816,29 @@ const DS = (() => {
     };
   }
 
+  // ── Signal registry for bidirectional sync ──
+  var _signalRegistry = {};
+  var _applyingRemoteDiff = false;
+
+  function _registerSignal(name, sig) {
+    _signalRegistry[name] = sig;
+  }
+
+  function _applyRemoteDiff(json) {
+    _applyingRemoteDiff = true;
+    try {
+      var data = JSON.parse(json);
+      for (var name in data) {
+        var sig = _signalRegistry[name];
+        if (sig) {
+          sig.value = data[name];
+        }
+      }
+      flush();
+    } catch(e) { console.warn('[ds-stream] bad diff:', e); }
+    _applyingRemoteDiff = false;
+  }
+
   function _initStream(url, mode) {
     _streamMode = mode || 'signal';
     _streamStart = performance.now();
@@ -1829,6 +1854,30 @@ const DS = (() => {
     };
     _streamWs.onclose = function() { setTimeout(function() { _initStream(url, mode); }, 2000); };
     console.log('[ds-stream] Source connected:', url, 'mode:', mode);
+
+    // Also open a receiver connection for bidirectional sync
+    var receiverUrl = url.replace(/\/source/, '/stream');
+    _initStreamReceiver(receiverUrl);
+  }
+
+  // Receiver WS — listens for diffs from other clients via relay
+  function _initStreamReceiver(url) {
+    var recvWs = new WebSocket(url);
+    recvWs.binaryType = 'arraybuffer';
+    recvWs.onmessage = function(e) {
+      if (!(e.data instanceof ArrayBuffer) || e.data.byteLength < HEADER_SIZE) return;
+      var bytes = new Uint8Array(e.data);
+      var view = new DataView(bytes.buffer);
+      var type = view.getUint8(0);
+      // Signal diff frame
+      if (type === 0x31) {
+        var payload = bytes.subarray(HEADER_SIZE);
+        var json = new TextDecoder().decode(payload);
+        _applyRemoteDiff(json);
+      }
+    };
+    recvWs.onclose = function() { setTimeout(function() { _initStreamReceiver(url); }, 2000); };
+    console.log('[ds-stream] Receiver connected:', url);
   }
 
   function _streamSend(type, flags, payload) {
@@ -1847,6 +1896,7 @@ const DS = (() => {
 
   function _streamDiff(name, value) {
     if (!_streamWs || _streamMode !== 'signal') return;
+    if (_applyingRemoteDiff) return; // prevent echo loops
     var obj = {};
     obj[name] = (typeof value === 'object' && value !== null && 'value' in value) ? value.value : value;
     _streamSend(0x31, 0, new TextEncoder().encode(JSON.stringify(obj)));
@@ -1896,6 +1946,12 @@ const DS = (() => {
       case 0x50:
         emit('remote_scroll', { dx: view.getInt16(0, true), dy: view.getInt16(2, true) });
         break;
+      case 0x31: {
+        // Signal diff — apply to local signals
+        var json = new TextDecoder().decode(payload);
+        _applyRemoteDiff(json);
+        break;
+      }
     }
   }
 
@@ -2107,7 +2163,7 @@ const DS = (() => {
            scene: scene, circle: circle, rect: rect, line: line,
            _initStream: _initStream, _streamDiff: _streamDiff, _streamSync: _streamSync,
            _streamSceneState: _streamSceneState, _connectStream: _connectStream,
-           _initWebRTC: _initWebRTC,
+           _initWebRTC: _initWebRTC, _registerSignal: _registerSignal,
            Signal: Signal, Derived: Derived, Effect: Effect, Spring: Spring };
   Object.defineProperty(_ds, '_streamWs', { get: function() { return _streamWs; } });
   Object.defineProperty(_ds, '_rtcDc', { get: function() { return _rtcDc; } });

examples/todo.ds

@@ -12,7 +12,7 @@ let filter_mode = "all"
 let total = len(todos)
 
 -- Stream: any device on the network can view / interact
-stream todo on "ws://localhost:9100" { mode: signal }
+stream todo on "ws://192.168.1.235:9100" { mode: signal }
 
 view app =
   column [