ui diff: Chunk frame diffs and group in report with thumbnails

[TheSecurityDev]

Description:

Does a proper diff on the replay video frame hashes using difflib to create groups of changes, and updates the HTML report to support showing these groups. Shows frame changes, additions, and deletions, and allows viewing the exact diff for each one.

It's still possible to see the full clips and diff like before, although we can consider whether the full diff will be needed in the future when we have these chunk diffs now.

Other changes:

The HTML report now supports video placeholders for no video slots (e.g. when frames added/removed) or as a loading indicator.

Also adds multithreading for creating the full diff video in the background while everything else is happening, hashing both video frames at once, and extracting the diff chunk frames in parallel.

Finally, cleans up the script and improves the logging, etc.

Example output:

============================================================
UI VIDEO DIFF REPORT
============================================================
Video 1:      /home/user/openpilot/selfdrive/ui/tests/diff/report/tizi_ui_replay.mp4
Video 2:      /home/user/openpilot/selfdrive/ui/tests/diff/report/tizi_ui_replay_2.mp4
HTML output:  diff.html
Diff video:   diff.mp4
Chunks dir:   diff-chunks

[1/5] Starting full video diff generation in background thread...
[2/5] Hashing frames...
Generating frame hashes for both videos...
  Found 870 frames in video 1 and 814 frames in video 2.
[3/5] Computing diff chunks...
  Found 9 diff chunk(s) with a total of 381 different frames.
[4/5] Extracting 9 diff chunk(s)...
  Getting video fps... 60.00 fps
  Processing 9 chunks with 8 threads...
Processing chunks: 100%|██████████████████████████████████████████████████████████████████| 9/9 [00:11<00:00,  1.30s/it]
[5/5] Generating HTML report...
  Results: ❌ Found 381 different frames out of 870 total (43.8%). Video 1 is longer by 56 frames.
  Report generated at: /home/user/openpilot/selfdrive/ui/tests/diff/report/diff.html
Waiting for diff video generation to finish...

Example video:

Note: This was generated by taking a replay I had lying around from a previous version, and for the second video, modifying the toggle color to blue, adding 2 seconds to the firehose page, and removing the experimental mode description scenario. So the diff is a lot messier than you might get in most realistic situations.

Recording.2026-02-22.190317.mp4

Here's a simpler example with just adding 2 more steps to the mici replay:

Identical example:

More comments:

• The diff chunks are stored by default in the diff-chunks folder under report, where diff matches the stem of the output name (e.g. mici-diff.html -> mici-diff-chunks). Thumbnails of the diff/new/removed clips are also extracted and used in the report.
• -nostdin is used for ffmpeg calls that run in threads because without it my terminal glitches out where I can't see the output as I type.
• Chunk padding support is left in the code, but I found I like it much better without it since it makes shorter diffs hard to see.

[github-actions]

This PR has had no activity for 24 days. It will be automatically closed in 7 days if there is no activity.

[TheSecurityDev]

Bump

[github-actions]

This PR has had no activity for 24 days. It will be automatically closed in 7 days if there is no activity.

[github-actions]

Process replay diff report

Replays driving segments through this PR and compares the behavior to master.
Please review any changes carefully to ensure they are expected.

✅ 0 changed, 66 passed, 0 errors

[github-actions]

This PR has had no activity for 24 days. It will be automatically closed in 7 days if there is no activity.

[TheSecurityDev]

I'll fix the merge conflicts

[github-actions]

This PR has had no activity for 24 days. It will be automatically closed in 7 days if there is no activity.

[TheSecurityDev]

Looks like we need #38514 to fix the errors