[Londopy]

Summary

• Adds native .rhs skin file import via drag-and-drop onto the game window
• New RhsImporter.cs extracts cursor, grid, colorset, hit sound, and composites background layers (with tint, opacity, placement modes) into a Rhythia skin folder
• Adds case "rhs": to the existing files_dropped handler in Rhythia.cs

What it does

Drop a .rhs file onto the game window and it:

1. Creates a new skin folder under skins/
2. Extracts cursor → game/cursor.png, border → game/grid.png
3. Extracts colorset → colorsets/{name}.txt
4. Composites all background layers (respecting canvas color, per-layer tint/opacity, screen/grid-relative placement, scale) into game/background.png and sets game_space = "void"
5. Extracts hit sound → hit.mp3
6. Writes config.toml and activates the skin immediately

Note skins are skipped since Rhythia uses 3D meshes instead of 2D textures.

[Londopy]

Closing — reopened as #198 targeting the indev branch per contributing guidelines.