feat(engine,producer,cli): capture failing dB/frame index on drawElement verify fallback

de_fallback_reason only told you the fallback happened (blank/psnr/oom/
capture_error), not the failing PSNR or frame index — that data existed as
text inside the thrown error's message and was discarded on the way to
telemetry. DrawElementVerificationError now carries structured
frameIndex/failedDb/verifyThresholdDb; the orchestrator reads them via the
new getDrawElementVerificationDetails helper instead of regexing message
text, and both telemetry surfaces (the render_complete perfSummary path and
the crash-survival RenderCaptureObservability mirror) emit
de_fallback_failed_db / de_fallback_frame_index.

Needed to distinguish "32dB vs the 32dB threshold, tune it" from "12dB real
corruption, investigate" during the parallel-router soak — currently that
distinction is invisible.

packages/engine/src/index.ts

@@ -91,6 +91,8 @@ export {
   captureFramesBatchPipelined,
   DrawElementVerificationError,
   isDrawElementVerificationError,
+  getDrawElementVerificationDetails,
+  type DrawElementVerificationDetails,
   recaptureDrawElementFrameForVerify,
   completeDeferredDrawElementInit,
   writeCapturedFrame,