build(lfs): track tests/*/src/*.png via Git LFS (#376)

## Summary

Track `tests/*/src/*.png` via Git LFS to mirror the existing policy for golden videos and `.mp4` fixtures.

## Why

`Chunk 11C` of `plans/hdr-followups.md`. Without this rule, regression suites that grow PNG fixtures over time would bloat the working-tree history and slow shallow clones.

## What changed

- `.gitattributes`: add `tests/*/src/*.png` to the LFS-tracked patterns.
- Migrates the six existing PNG fixtures (1.6 MB combined: `hdr-photo-pq.png` plus `heygen-promo-preview-assets/` screenshots) onto LFS in the same commit so the rule applies retroactively.

## Test plan

- [x] `git lfs ls-files` includes the HDR PNG fixtures after commit.
- [x] Working tree size for these files goes from 1.6 MB to 6 × ~130 B LFS pointers.

## Stack

Chunk 11C of `plans/hdr-followups.md`. Independent of all code changes.

packages/engine/src/services/videoFrameExtractor.ts

@@ -418,6 +418,7 @@ export async function extractAllVideoFrames(
 
   const hdrInfo = analyzeCompositionHdr(videoColorSpaces);
   if (hdrInfo.hasHdr && hdrInfo.dominantTransfer) {
+
     // dominantTransfer is "majority wins" — if a composition mixes PQ and HLG
     // sources (rare but legal), the minority transfer's videos get converted
     // with the wrong curve. We treat this as caller-error: a single composition