fix(producer,engine): gate drawelement on ancestor background-image + tail verify sample

packages/engine/src/services/frameCapture.ts

@@ -3107,6 +3107,19 @@ export async function getCompositionDuration(session: CaptureSession): Promise<n
  * init-time screenshot → the render falls back to the screenshot path (slower,
  * never wrong). Cost when passing: ~K×(seek+screenshot) ≈ 150–300ms at init.
  */
+/**
+ * Timeline fractions the self-verify samples. First k-1 evenly spaced, last
+ * pinned at 95%: late-onset damage (an end-of-comp reveal exposing pixels DE
+ * paints wrong) was invisible to the old (i+1)/(k+1) grid, whose final sample
+ * sat at 80% — measured miss: a body-gradient drop starting at ~79% of the
+ * timeline passed verification while the drained output bottomed at 30.9 dB.
+ */
+export function computeDeVerifySampleFractions(k: number): number[] {
+  if (k <= 0) return [];
+  if (k === 1) return [0.95];
+  return [...Array.from({ length: k - 1 }, (_, i) => (i + 1) / k), 0.95];
+}
+
 async function captureDeVerificationFrames(
   session: CaptureSession,
   page: Page,
@@ -3153,7 +3166,7 @@ async function captureDeVerificationFrames(
   // render (the detectCssEffectRisk lesson), and because DE frames and truth
   // would share the corruption, PSNR would pass on the damaged output.
   // Seeking 0 → ascending reproduces the render's own seek order.
-  const fractions = Array.from({ length: k }, (_, i) => (i + 1) / (k + 1));
+  const fractions = computeDeVerifySampleFractions(k);
   const seekTo = async (t: number): Promise<void> => {
     await page.evaluate((tt: number) => {
       const hf = (