feat(distributed): add optional cfr flag for exact constant frame rate

Distributed-render output today uses -c:v copy through concat → mux →
faststart, which means PTS timestamps from each chunk pass through
unchanged. Container r_frame_rate is exact (#1040 + this PR's parent),
but stream-level avg_frame_rate stays PTS-derived and can land on
fractional rationals like 27648000/921677 over a 60s render. Same for
sub-ms duration drift.

This is the achievable bar within -c copy stream-copy concat. For most
consumers (browser playback, YouTube, etc.) the difference is invisible.
For downstream tools that strict-check avg_frame_rate or
ms-precision duration (broadcast workflows, frame-accurate compositors,
some third-party transcoders), it matters.

Adds an opt-in cfr config flag (default false). When true, the
assemble step's final pass re-encodes with -fps_mode cfr -r <fps>
instead of -c copy, producing exact CFR output. Trade-off: ~2-5x the
stitch time for a 60s 1080p clip; second-generation H.264 quality loss
is negligible at -crf 18 but is non-zero.

packages/aws-lambda/src/events.ts

@@ -81,6 +81,16 @@ export interface AssembleEvent {
   OutputS3Uri: string;
   /** Output container format; drives file vs frame-dir handling. */
   Format: DistributedFormat;
+  /**
+   * Optional exact-CFR re-encode at assemble time. When `true`, the final
+   * assembled video is re-encoded with `-fps_mode cfr -r <fps>` so the
+   * stream's `avg_frame_rate` matches the container's `r_frame_rate`
+   * exactly (and the file's duration is exact, not PTS-derived). Trade-off
+   * is ~2-5x the assemble wall-clock. mp4 only — webm / mov stream-copy
+   * paths already produce exact avg_frame_rate. Default `false` /
+   * unset preserves current `-c copy` behavior.
+   */
+  Cfr?: boolean;
 }
 
 /**