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/handler.ts

@@ -407,7 +407,9 @@ async function handleAssemble(
         ? join(work, "output-frames")
         : join(work, `output${formatExtension(event.Format)}`);
 
-    const result: AssembleResult = await primitive(planDir, chunkPaths, audioPath, finalOutput);
+    const result: AssembleResult = await primitive(planDir, chunkPaths, audioPath, finalOutput, {
+      cfr: event.Cfr === true,
+    });
 
     if (event.Format === "png-sequence") {
       const tarball = `${finalOutput}.tar.gz`;