Function validate 

pub fn validate(
    engine: &GameEngine,
    player: PlayerId,
    action: &Action,
) -> Result<(), CardinalError>

Validate that an action is legal in the current game state. Checks:

• Only the priority player can pass priority
• Only the active player can take other actions
• The current phase allows actions
• Stack requirements are met (if action requires empty stack)
• Zone ownership and card ownership are valid