feat(producer): parity telemetry gate for per-clip frame-count invariant

Field signals ts=1784139267 (win32 15-injection blank later clips,
injection-count-scaled) + ts=1784144554 (darwin 147-authored-clip
visibility gap, authored-clip-count-scaled). check/snapshot passes,
final MP4 has blank/missing clips. Extends #2474's videoCount:0 probe
with per-clip captured-vs-expected frame ratio + threshold fail-loud
gate at render finalization.

Stack: PR #4 of 9 (base via/darwin-goto-nav-timeout-hint).
Signed-off-by: Via <via@heygen.com>

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

@@ -109,6 +109,28 @@ export interface RenderExtractionObservability {
   vfrPreflightCount?: number;
   cacheHits?: number;
   cacheMisses?: number;
+  /**
+   * Per-clip captured-vs-expected-frame gauges. Emitted by the parity gate
+   * at extract finalization (see `videoFrameCoverage.ts`). Undefined when
+   * the render has no source videos to cover.
+   *
+   * • `minVideoFrameCoverageRatio` — worst clip's `captured / expected`
+   *   ratio (0 when a clip was never extracted; a strong "later-injected
+   *   clip silently dropped" signal per field ts=1784139267).
+   * • `coverageShortfallClipCount` — clips whose ratio fell below the
+   *   configured threshold (`HF_VIDEO_COVERAGE_THRESHOLD`, default 0.95).
+   *   Non-zero only ever accompanies a `VideoFrameCoverageError` throw.
+   */
+  minVideoFrameCoverageRatio?: number;
+  coverageShortfallClipCount?: number;
+  /**
+   * Count of authored `[data-start]` clip windows in the compiled HTML —
+   * a coarse proxy for the ts=1784144554 field signal shape (147-clip
+   * composition, 130 word-level caption divs authored-clip-count-scaled
+   * failure). Static scan; dynamic script-inserted timed clips land in
+   * the probe-stage's `hasRuntimeInsertedMedia` path (PR #2474).
+   */
+  authoredTimedClipCount?: number;
 }
 
 export interface RenderInitObservability {