Added "s&box" to the Game Engine collection.

[gunnar-hensel]

Please confirm this pull request meets the following requirements:

• I followed the contributing guidelines: https://github.com/github/explore/blob/main/CONTRIBUTING.md.
• This change is not self-promotion.

Which change are you proposing?

• Suggesting edits to an existing topic or collection
• Curating a new topic or collection
• Something that does not neatly fit into the binary options above

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Editing an existing topic or collection

I'm suggesting these edits to an existing topic or collection:

• Image (and my file is *.png, square, dimensions 288x288, size <= 75 kB)
• Content (and my changes are in index.md)

Added Facepunch's open source Source 2 engine "s&box" due to it's recent major spotlight.
Repo: github.com/Facepunch/sbox-public
Official website: sbox.game

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[Copilot]

Pull request overview

Adds Facepunch’s s&box repository to the existing Game Engines collection, expanding the curated list of engine/framework repositories.

Changes:

• Added Facepunch/sbox-public to collections/game-engines/index.md.
• Removed trailing whitespace from the B4uti4github/choppy-js entry (no functional change).

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