Implement `animation-play-state` support

[FlorianRappl]

Description
Animation pause/resume semantics are missing.

Scope

• Support animation-play-state: running | paused.
• Ensure paused animations freeze at current progress and resume correctly.
• Apply to single and multi-animation declarations.

Acceptance criteria

• Paused animations stop advancing.
• Toggling paused/running resumes from the same position.
• Regression tests cover per-animation play-state behavior.