feat: beats viewer, score sounds, audio early-exit guards

- New beats-viewer.ds: step sequencer spectator via relay stream
- game-pong.ds: added score sound effects (220Hz/440Hz sawtooth)
- Runtime: _playTone skips when freq<=0, _playNoise skips when dur<=0
- All 47 examples compile, 136 tests pass

compiler/ds-codegen/src/js_emitter.rs

@@ -3400,6 +3400,7 @@ const DS = (() => {
   }
 
   function _playTone(freq, durationMs, type) {
+    if (!freq || freq <= 0 || !durationMs || durationMs <= 0) return;
     var ctx = _ensureAudio();
     var osc = ctx.createOscillator();
     var gain = ctx.createGain();
@@ -3414,6 +3415,7 @@ const DS = (() => {
   }
 
   function _playNoise(durationMs, vol) {
+    if (!durationMs || durationMs <= 0) return;
     var ctx = _ensureAudio();
     var bufSize = ctx.sampleRate * (durationMs / 1000);
     var buf = ctx.createBuffer(1, bufSize, ctx.sampleRate);

examples/beats-viewer.ds

@@ -0,0 +1,72 @@
+-- DreamStack Beats Viewer (Spectator)
+-- Watches the step sequencer live via streaming relay.
+-- Shows the beat grid state from a different browser tab.
+--
+-- Run with:
+--   Tab 1: cargo run -p ds-stream                              (relay on :9100)
+--   Tab 2: dreamstack dev examples/step-sequencer.ds            (player)
+--   Tab 3: dreamstack dev examples/beats-viewer.ds --port 3001  (viewer)
+
+import { Badge } from "../registry/components/badge"
+
+-- Connect to the beats stream
+let beats = stream from "ws://localhost:9100/stream/beats"
+
+view viewer = column [
+  text "👁️ Beats Spectator" { variant: "title" }
+  text "Watching the step sequencer live via relay" { variant: "subtitle" }
+
+  row [
+    Badge { label: "LIVE 🔴", variant: "error" }
+    Badge { label: "BPM: {beats.bpm}", variant: "info" }
+    Badge { label: "Step: {beats.step}", variant: "warning" }
+  ]
+
+  -- Playhead indicator
+  row [
+    text "     " { variant: "muted" }
+    for i in [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] ->
+      text (if i == beats.step then "▼" else "·") { variant: "muted" }
+  ]
+
+  -- Kick row (read-only)
+  row [
+    text "KICK " { variant: "caption" }
+    for i in [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] ->
+      text (if beats.kick then "●" else "○") {
+        variant: (if i == beats.step then "warning" else "muted")
+      }
+  ]
+
+  -- Snare row
+  row [
+    text "SNRE " { variant: "caption" }
+    for i in [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] ->
+      text (if beats.snare then "●" else "○") {
+        variant: (if i == beats.step then "warning" else "muted")
+      }
+  ]
+
+  -- HiHat row
+  row [
+    text "HHAT " { variant: "caption" }
+    for i in [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] ->
+      text (if beats.hihat then "●" else "○") {
+        variant: (if i == beats.step then "warning" else "muted")
+      }
+  ]
+
+  -- Bass row
+  row [
+    text "BASS " { variant: "caption" }
+    for i in [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] ->
+      text (if beats.bass then "●" else "○") {
+        variant: (if i == beats.step then "warning" else "muted")
+      }
+  ]
+
+  when beats.playing -> text "▶ Playing" { variant: "muted" }
+  else -> text "⏸ Paused" { variant: "muted" }
+
+  text "View-only • beat pattern received via relay" { variant: "muted" }
+]

examples/game-pong.ds

@@ -65,6 +65,10 @@ every 33 -> bvy = if ballX > 604 then -2 else bvy
 every 33 -> play_tone(if ballX < 26 then (if ballY > p1y then (if ballY < p1y + 80 then 880 else 0) else 0) else 0, 60)
 every 33 -> play_tone(if ballX > 566 then (if ballY > p2y then (if ballY < p2y + 80 then 660 else 0) else 0) else 0, 60)
 
+-- Sound effects: scoring
+every 33 -> play_tone(if ballX < -8 then 220 else 0, 200, "sawtooth")
+every 33 -> play_tone(if ballX > 604 then 440 else 0, 200, "sawtooth")
+
 -- Stream for viewers
 stream pong on "ws://localhost:9100/peer/pong" {
   mode: signal,