feat: synthesize trajectory analysis windows

packages/acoustics-core/signal.js

@@ -15,7 +15,16 @@ function sampleAsset(signal, assetBuffers, time) {
   return channel[left] + (rightValue - channel[left]) * fraction;
 }
 
-export function renderSignal(entry, scene, assetBuffers = new Map(), seedSalt = 0) {
+/**
+ * Render one signal entry into the scene-sized output buffer.
+ *
+ * `timeMapping` lets a short, rebased scene window retain the original signal
+ * timeline. `sceneSampleOffset` maps output sample zero to an absolute sample
+ * in the original scene, while `signalStartTime`/`signalStartSample` preserve
+ * the entry's original tone/asset time origin and deterministic noise index.
+ * Omitting it keeps the historical whole-scene behaviour exactly unchanged.
+ */
+export function renderSignal(entry, scene, assetBuffers = new Map(), seedSalt = 0, timeMapping = {}) {
   const sampleCount = Math.ceil(scene.duration * scene.sampleRate);
   const output = new Float32Array(sampleCount);
   const startTime = entry.startTime ?? 0;
@@ -23,12 +32,25 @@ export function renderSignal(entry, scene, assetBuffers = new Map(), seedSalt =
   const gain = dbToLinear(entry.gainDb ?? 0);
   const startSample = Math.max(0, Math.ceil(startTime * scene.sampleRate));
   const endSample = Math.min(sampleCount, Math.ceil((startTime + duration) * scene.sampleRate));
+  const sceneSampleOffset = timeMapping.sceneSampleOffset ?? 0;
+  const signalStartTime = timeMapping.signalStartTime ?? startTime;
+  const signalStartSample = timeMapping.signalStartSample ?? Math.ceil(startTime * scene.sampleRate);
+  if (!Number.isSafeInteger(sceneSampleOffset) || sceneSampleOffset < 0) {
+    throw new RangeError("sceneSampleOffset must be a non-negative safe integer.");
+  }
+  if (!Number.isFinite(signalStartTime)) {
+    throw new TypeError("signalStartTime must be finite.");
+  }
+  if (!Number.isSafeInteger(signalStartSample) || signalStartSample < 0) {
+    throw new RangeError("signalStartSample must be a non-negative safe integer.");
+  }
   const signal = entry.signal;
   const seed = (scene.seed + (signal.seedOffset ?? 0) + hashString(entry.id ?? "signal") + seedSalt) >>> 0;
 
   for (let sampleIndex = startSample; sampleIndex < endSample; sampleIndex += 1) {
-    const localIndex = sampleIndex - startSample;
-    const localTime = sampleIndex / scene.sampleRate - startTime;
+    const absoluteSampleIndex = sceneSampleOffset + sampleIndex;
+    const localIndex = absoluteSampleIndex - signalStartSample;
+    const localTime = absoluteSampleIndex / scene.sampleRate - signalStartTime;
     let value;
     if (signal.type === "tone") {
       value = Math.sin(TAU * signal.frequency * localTime + ((signal.phaseDeg ?? 0) * Math.PI) / 180);