fix: handle NaN in zero-scan fallback and symmetric divergence check

- validateDuration returns 0 instead of NaN when both container is
  NaN and scanned is zero
- Use Math.abs for divergence check so container under-reporting is
  also corrected (not just over-reporting)

src/lib/exporter/streamingDecoder.test.ts

@@ -27,9 +27,17 @@ describe("validateDuration", () => {
 		expect(validateDuration(15.0, 15.3)).toBe(15.0);
 	});
 
+	it("returns scanned duration when container under-reports beyond threshold", () => {
+		expect(validateDuration(10, 15.3)).toBe(15.3);
+	});
+
 	it("returns container duration when scanned is zero (corrupted/empty file)", () => {
 		expect(validateDuration(10, 0)).toBe(10);
 	});
+
+	it("returns 0 when both container is NaN and scanned is zero", () => {
+		expect(validateDuration(NaN, 0)).toBe(0);
+	});
 });
 
 describe("shouldFailDecodeEndedEarly", () => {

src/lib/exporter/streamingDecoder.ts

@@ -86,12 +86,12 @@ export function validateDuration(containerDuration: number, scannedDuration: num
 	if (scannedDuration <= 0) {
 		// Zero scanned duration means corrupted/empty file — fall back to container
 		// (downstream shouldFailDecodeEndedEarly will catch truly empty files)
-		return Math.max(containerDuration, 0);
+		return Number.isFinite(containerDuration) ? Math.max(containerDuration, 0) : 0;
 	}
 	if (!Number.isFinite(containerDuration) || containerDuration <= 0) {
 		return scannedDuration;
 	}
-	if (containerDuration - scannedDuration > DURATION_DIVERGENCE_THRESHOLD_SEC) {
+	if (Math.abs(containerDuration - scannedDuration) > DURATION_DIVERGENCE_THRESHOLD_SEC) {
 		return scannedDuration;
 	}
 	return containerDuration;