feat: explicit signal output API for stream declarations

Add 'output: field1, field2' syntax to stream declarations to control
which signals are exposed over the relay. Only listed signals are
registered in _signalRegistry (and thus streamed). Omitting output
streams all signals (backwards-compatible).

Also strips internal sync metadata (_pid, _v) from receiver state
so composition consumers only see clean signal values.

Parser: parse comma-separated idents after 'output:' key
AST: Vec<String> output field on StreamDecl
Codegen: conditional _registerSignal, delete _pid/_v on receive

Example: stream counter on 'ws://...' { mode: signal, output: count, doubled }

compiler/ds-codegen/src/js_emitter.rs

@@ -78,6 +78,13 @@ impl JsEmitter {
         self.indent += 1;
 
         // Phase 1: Create all signals
+        // Collect explicit output list from stream declaration (if any)
+        let stream_outputs: Vec<String> = program.declarations.iter()
+            .filter_map(|d| if let Declaration::Stream(s) = d { Some(s) } else { None })
+            .flat_map(|s| s.output.iter().cloned())
+            .collect();
+        let has_explicit_output = !stream_outputs.is_empty();
+
         self.emit_line("// ── Signals ──");
         for node in &graph.nodes {
             match &node.kind {
@@ -110,8 +117,10 @@ 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));
+                    // Register for streaming only if in output list (or no explicit output = all)
+                    if !has_explicit_output || stream_outputs.contains(&node.name) {
+                        self.emit_line(&format!("DS._registerSignal(\"{}\", {});", node.name, node.name));
+                    }
                 }
                 SignalKind::Derived => {
                     // Find the let declaration to get the expression
@@ -122,8 +131,10 @@ impl JsEmitter {
                             "const {} = DS.derived(() => {});",
                             node.name, js_expr
                         ));
-                        // Register derived signal so it's included in stream sync
-                        self.emit_line(&format!("DS._registerSignal(\"{}\", {});", node.name, node.name));
+                        // Register for streaming only if in output list (or no explicit output = all)
+                        if !has_explicit_output || stream_outputs.contains(&node.name) {
+                            self.emit_line(&format!("DS._registerSignal(\"{}\", {});", node.name, node.name));
+                        }
                     }
                 }
                 SignalKind::Handler { .. } => {} // Handled later
@@ -2037,6 +2048,9 @@ const DS = (() => {
           case 0x31: // SignalDiff — partial state update
             try {
               var newState = JSON.parse(new TextDecoder().decode(pl));
+              // Strip internal sync metadata
+              delete newState._pid;
+              delete newState._v;
               if (type === 0x30) {
                 state.value = newState; // full replace — new object
               } else {

compiler/ds-parser/src/ast.rs

@@ -113,6 +113,8 @@ pub struct StreamDecl {
     pub relay_url: Expr,
     pub mode: StreamMode,
     pub transport: StreamTransport,
+    /// Explicit signal output list. Empty = stream all signals (backwards-compat).
+    pub output: Vec<String>,
     pub span: Span,
 }
 

compiler/ds-parser/src/parser.rs

@@ -365,7 +365,7 @@ impl Parser {
         }))
     }
 
-    /// Parse: `stream <view_name> on <url_expr> { mode: pixel | delta | signal }`
+    /// Parse: `stream <view_name> on <url_expr> { mode: signal, output: a, b }`
     fn parse_stream_decl(&mut self) -> Result<Declaration, ParseError> {
         let line = self.current_token().line;
         self.advance(); // consume `stream`
@@ -380,9 +380,10 @@ impl Parser {
 
         let relay_url = self.parse_expr()?;
 
-        // Optional mode block: `{ mode: signal, transport: webrtc }`
+        // Optional config block: `{ mode: signal, transport: webrtc, output: a, b }`
         let mut mode = StreamMode::Signal;
         let mut transport = StreamTransport::WebSocket;
+        let mut output: Vec<String> = Vec::new();
 
         if self.check(&TokenKind::LBrace) {
             self.advance(); // {
@@ -410,6 +411,24 @@ impl Parser {
                         }
                         _ => return Err(self.error("Expected webrtc or websocket".into())),
                     }
+                } else if key == "output" {
+                    // Parse comma-separated identifiers: output: count, doubled
+                    loop {
+                        let name = self.expect_ident()?;
+                        output.push(name);
+                        if self.check(&TokenKind::Comma) {
+                            self.advance();
+                            // Peek ahead: if next is a known key or RBrace, stop
+                            // (the comma was a field separator, not an output separator)
+                            match self.peek() {
+                                TokenKind::RBrace => break,
+                                TokenKind::Ident(s) if s == "mode" || s == "transport" || s == "output" => break,
+                                _ => {} // continue parsing output names
+                            }
+                        } else {
+                            break;
+                        }
+                    }
                 }
                 if self.check(&TokenKind::Comma) { self.advance(); }
                 self.skip_newlines();
@@ -422,6 +441,7 @@ impl Parser {
             relay_url,
             mode,
             transport,
+            output,
             span: Span { start: 0, end: 0, line },
         }))
     }

examples/streaming-counter.ds

@@ -10,7 +10,7 @@ let count = 0
 let doubled = count * 2
 let message = "Streaming Counter"
 
-stream counter on "ws://localhost:9100" { mode: signal }
+stream counter on "ws://localhost:9100" { mode: signal, output: count, doubled }
 
 view counter =
   column [