feat(animation): impl Animatable for Rot2

[YurikoDX]

Objective

• Enable animation support for 2D rotations.
• Currently, Rot2 does not implement the Animatable trait, which prevents UiTransform::rotation and other 2D components from being animated via AnimationGraph.

Solution

• Implemented Animatable for Rot2 in bevy_animation.
• Implementation Parity: Followed the exact pattern of the existing Quat implementation to ensure consistency across the engine's math types:
  • Used slerp for interpolate to ensure shortest-path 2D rotation.
  • Implemented blend with cumulative SLERP to support multi-track and additive blending.
  • Added documentation comments consistent with Quat.

Testing

• Example Validation: Verified by adding a rotation curve to the official animated-ui example.
• Environment: Tested on Linux (NVIDIA MX150 / Intel UHD 620).

---

Showcase

Modified the animated-ui example to include UiTransform::rotation animation:

Click to view code snippet

use std::f32::consts::TAU;

animation_clip.add_curve_to_target(
    animation_target_id,
    AnimatableCurve::new(
        animated_field!(UiTransform::rotation),
        AnimatableKeyframeCurve::new(
            [0.0, 1.0, 2.0, 3.0]
                .into_iter()
                .zip([0., TAU / 3., TAU / 1.5, TAU].map(Rot2::radians))
        )
        .expect("should be able to build rotation curve")
    ),
);

[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-23618

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.

[alice-i-cecile]

Would not mind a test or two, especially for the shortest path behavior, but this LGTM. Thanks!

You need to format this code (see CI), and we'll need a second approval before we can merge though.

[alice-i-cecile]

This needs to target main

Example Validation: Verified by adding a rotation curve to the official animated-ui example.

This was also not done in this PR.

[alice-i-cecile]

Swapping this to target the main branch is non-trivial. You're probably better off just cherrypicking your changes into a new branch and opening a new PR.

[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-23618

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.

[YurikoDX]

I've cherry-picked these changes into a clean branch targeting main as suggested! Closing this in favor of the new PR: #23621

Thanks again for the guidance, @alice-i-cecile !