Add Barbell Squat runner

[qyuja]

Pull Request

Type of Change

• Add a new runner
• Fix or improve an existing runner
• Other (site, docs, tooling)

Summary

Adds barbell-squat, an original monochrome rear-view barbell squat animation. The eight-frame loop moves from standing through a deep squat and mirrors the descent frames for a stable return to lockout.

Checklist for Adding a Runner

• I have read CONTRIBUTING.md and followed the steps described there.
• The runner is my original work, or I hold the rights to it, and I consent to its distribution under this repository's license (Apache-2.0).
• The runner does NOT contain third-party intellectual property (e.g., Pokémon, Mario, or other copyrighted/trademarked characters).
• Files are placed at runners/<runner-name>/ as <runner-name>-frames.zip, metadata.json, and preview.png.
• All frames are PNG files named <runner-name>-frame-<N>.png (numbered from 0), with a height of exactly 36px, a width between 10px and 100px, and all frames share the same size.
• The zip contains only the frames — no .DS_Store, __MACOSX/, or ._* entries.
• metadata.json contains the correct author, displayName, type (monochrome or color), and tags.
• preview.png is an animated PNG (APNG), 36px tall, at most 100px wide, and animates correctly.
• runners/manifest.json includes the new runner name and stays sorted alphabetically.

Validation

python3 .github/scripts/validate_runners.py --all

All 43 runners pass validation.