Python 3.14 support, new env variant, and stability updates

Changes since v1.10.2

Compatibility

• Added Python 3.14 support.
• Dropped Python 3.9 support.
• Fixed NumPy 2.0 compatibility.
• Modernized build configuration.

CI and build tooling

• Added CI/install support for pygame system dependencies.
• Installed SDL2/freetype/portmidi packages for docs/build environment.

Bug fixes

• Fixed SubprocVecEnv ConnectionResetError with forkserver/spawn.
• Fixed zero division in LinearSpline2D.
• Fixed divide-by-zero warning in LidarObservation.
• Fixed incorrect coordinate transformation in spline.

Features and docs

• Added a generic version of the roundabout environment.
• Refreshed docs branding and added mise/just tooling.
• Updated docs workflows and README content.
• Added multiple paper citations / references.