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.

.github/workflows/ci.yml

@@ -45,6 +45,8 @@ jobs:
     timeout-minutes: 10
     steps:
       - uses: actions/checkout@v4
+        with:
+          lfs: true
       - uses: oven-sh/setup-bun@v2
       - uses: actions/setup-node@v4
         with:
@@ -60,6 +62,8 @@ jobs:
     timeout-minutes: 5
     steps:
       - uses: actions/checkout@v4
+        with:
+          lfs: true
       - uses: oven-sh/setup-bun@v2
       - uses: actions/setup-node@v4
         with:
@@ -73,6 +77,8 @@ jobs:
     timeout-minutes: 5
     steps:
       - uses: actions/checkout@v4
+        with:
+          lfs: true
       - uses: oven-sh/setup-bun@v2
       - uses: actions/setup-node@v4
         with:
@@ -88,6 +94,8 @@ jobs:
     timeout-minutes: 10
     steps:
       - uses: actions/checkout@v4
+        with:
+          lfs: true
       - uses: oven-sh/setup-bun@v2
       - uses: actions/setup-node@v4
         with:
@@ -104,6 +112,8 @@ jobs:
     timeout-minutes: 10
     steps:
       - uses: actions/checkout@v4
+        with:
+          lfs: true
       - uses: oven-sh/setup-bun@v2
       - uses: actions/setup-node@v4
         with:
@@ -119,6 +129,8 @@ jobs:
     timeout-minutes: 10
     steps:
       - uses: actions/checkout@v4
+        with:
+          lfs: true
       - uses: oven-sh/setup-bun@v2
       - uses: actions/setup-node@v4
         with:

.github/workflows/windows-render.yml

@@ -60,6 +60,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           ref: ${{ github.event.inputs.ref }}
+          lfs: true
 
       - name: Show platform info
         shell: pwsh
@@ -222,6 +223,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           ref: ${{ github.event.inputs.ref }}
+          lfs: true
 
       # -----------------------------------------------------------------
       # Install FFmpeg so vitest suites that gate on `HAS_FFMPEG`