fix: expose physical aliasing and delayed arrivals

apps/acoustic-camera/source-selection.js

@@ -0,0 +1,41 @@
+import {
+  describeFrequencyAliasing,
+  observeFrequencyBand,
+} from "/packages/acoustics-core/sampling.js";
+
+/**
+ * Select evaluation truth sources for one observed band.
+ *
+ * Arrival ids come from synthesis path timing and take precedence over the
+ * legacy emission-time fallback. This helper is evaluation-only; localization
+ * never receives truth positions or ids.
+ */
+export function sourcesForObservedBand(
+  scene,
+  band,
+  fftSize,
+  options = {},
+) {
+  const observation = observeFrequencyBand(band, scene.sampleRate, fftSize);
+  const observedBins = new Set(observation.bins);
+  const arrivingSourceIdSet = Array.isArray(options.arrivingSourceIds)
+    ? new Set(options.arrivingSourceIds)
+    : null;
+  const time = options.time ?? null;
+  return scene.sources.filter((source) => {
+    if (arrivingSourceIdSet) {
+      if (!arrivingSourceIdSet.has(source.id)) return false;
+    } else if (time !== null) {
+      const startTime = source.startTime ?? 0;
+      const endTime = startTime + (source.duration ?? scene.duration);
+      if (time < startTime || time >= endTime) return false;
+    }
+    if (source.signal.type !== "tone") return true;
+    const analogInside =
+      source.signal.frequency >= band.minHz && source.signal.frequency <= band.maxHz;
+    if (analogInside) return true;
+    const alias = describeFrequencyAliasing(source.signal.frequency, scene.sampleRate);
+    const nearestAdcBin = Math.round(alias.adcFrequencyHz / observation.binWidthHz);
+    return observedBins.has(nearestAdcBin);
+  });
+}