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/producer/src/services/render/observability.ts

@@ -60,6 +60,10 @@ export interface RenderCaptureObservability {
    * telemetry from one that never attempted any fallback.
    */
   deFallbackReason?: string;
+  /** The failing PSNR (dB) when `deFallbackReason === "psnr"`; undefined for blank/oom/capture_error (no score exists). */
+  deFallbackFailedDb?: number;
+  /** Frame index the verification failure was detected at; set for both "psnr" and "blank" fallback reasons. */
+  deFallbackFrameIndex?: number;
   /** Auto-parallel inversion outcome: "inverted" (fired, held) | "reverted" (fired, self-verify retry rolled back). */
   deWorkerInversion?: "inverted" | "reverted";
   /** Worker count the resolver would have used absent the inversion; undefined if it never fired. */