Fixes #9179 – Implements Multi-Stage Interpolation Chains with Blend Modes

This PR extends Kivy’s Animation system by adding support for multi-stage interpolation chains with configurable blend modes. Previously, Animation only supported single transition functions, which limited complex motion sequences.

Key Changes:

New Methods:

• chain_interpolators(interpolators, weights=None, blend_mode='linear') → creates a callable for multiple transitions.
• get_chain_value(progress, interpolators, blend_mode='linear') → evaluates the chain at any progress.
• validate_chain(interpolators) → validates chain structure, detecting gaps, overlaps, and invalid transitions.

Blend Modes Implemented: linear, weighted, multiplicative, overlay, screen.

Features:

• Supports chaining of easing functions like elastic, bounce, cubic, sine, quad, etc.
• Properly clamps progress, handles empty chains, and ensures segment continuity.

Behavior:

• Complex animations can now be defined by sequencing multiple easing functions with offsets and blending.
• Gaps in the chain raise ValueError; overlaps use the first matching range.
• Weighted blending validates that weights sum to 1.0.

Testing:

Added unit tests for:

• Valid and invalid chains
• Each blend mode
• Edge cases for progress outside [0,1]
• Empty chains

Documentation:

• Docstrings added for all new functions.
• Examples provided in Animation docstring.

Maintainer Merge Checklist:

• Title is descriptive/clear for inclusion in release notes.
• Applied a Component: Animation label.
• Applied the api-deprecation or api-break label.
• Applied the release-highlight label to be highlighted in release notes.
• Added to the milestone version it was merged into.