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/chunkEncoder.test.ts

@@ -439,6 +439,7 @@ describe("buildEncoderArgs HDR color space", () => {
     expect(args[paramIdx + 1]).not.toContain("max-cll");
   });
 
+
   it("strips HDR and tags as SDR/BT.709 when codec=h264 (libx264 has no HDR support)", () => {
     // libx264 cannot encode HDR. Rather than emit a "half-HDR" file (BT.2020
     // container tags + BT.709 VUI inside the bitstream — confusing to HDR-aware