[Feature] Animation blending

[vmarcella]

Overview

Add the ability to blend between multiple animations for smooth transitions
and layered animation.

Current State

No response

Scope

Goals:

• Crossfade between two animations
• Blend by weight (0.0 to 1.0)
• Additive blending (layer on top)
• Blend over time (transition duration)

Non-Goals:

• Animation state machine
• Per-bone masking
• Sync groups

Proposed API

pub struct AnimationBlender {
  animations: Vec<(AnimationPlayer, f32)>, // (player, weight)
}

impl AnimationBlender {
  pub fn new() -> Self;
  pub fn add(&mut self, player: AnimationPlayer, weight: f32);
  pub fn set_weight(&mut self, index: usize, weight: f32);
  pub fn crossfade(&mut self, from: usize, to: usize, duration: f32);
  pub fn update(&mut self, dt: f32);
  pub fn apply_to_pose(&self, skeleton: &Skeleton, pose: &mut Pose);
}

impl AnimationPlayer {
  pub fn blend_with(&self, other: &Self, factor: f32, pose: &mut Pose);
}

Acceptance Criteria

• Two animations blend by weight
• Weight 0.0 shows only first, 1.0 only second
• Crossfade smoothly transitions
• Additive blending layers effects
• Weights can be animated over time

Affected Crates

lambda-rs

Notes

• Blend transforms (position lerp, rotation slerp)
• Normalize weights if they don't sum to 1.0