Implement Manim 2D backend for CBFAnimator

[Eduard-Zippenfenig]

Summary

Implements the 2D Manim backend for CBFAnimator, filling the gap where backend="manim" was accepted but raised NotImplementedError ("Manim 2D backend not yet implemented").

CBFAnimator(states, backend="manim") (or "manim-<low|medium|high|production>") now renders the full declarative element API — goals, circular/elliptical obstacles, line/scatter trajectories, agents with bodies/safety zones/solid+dashed trails, per-frame linear and data predictions, and the time overlay — to MP4 (or GIF) via Manim.

Demo

Rendered from the README quick-start CBF example (unicycle reach-avoid) with backend="manim-medium":

The existing high-level path works as a drop-in: visualize_crowd(..., backend="manim") renders pedestrians with safety zones and fading predictions with no other changes.

Design

Mirrors the existing 3D module (manim_3d_multi_robot.py) throughout:

• Guarded try: import manim with Scene = object fallback — the module imports cleanly when the manim extra is absent (as in CI), and the constructor raises the standard cbfkit[manim] ImportError via _require_manim().
• Data injected into CBFAnimator2DScene via class attributes before scene.render(); animation driven by a ValueTracker + updaters.
• Reuses the established manim-<quality> backend-string convention and _parse_manim_backend — no new config fields. Invalid quality strings raise ValueError before the dependency check.
• White canvas so matplotlib-convention colors (e.g. black obstacles) stay visible; matplotlib-style color names translated to hex.
• Render intermediates go to a temp dir, so the caller's cwd stays clean; the finished video is copied to the requested path.
• _ManimMixin added to CBFAnimator alongside the matplotlib/plotly mixins; dispatch added to build/animate/save/show.

Also adds a [mypy-manim.*] stanza to mypy.ini.

Tests

• Dependency-free (run in CI without manim): quality parsing for all five backend strings, import-guard ImportError, invalid-quality ValueError, and save() dispatch with a mocked render.
• A real-render smoke test that skips when manim is not installed.
• Full suite: 479 passed, 4 skipped locally (Python 3.11, manim 0.20.1); ruff check src clean; changed files formatted with black 25.11.0 (pre-commit pin).

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