Choreograph: when field + --role flag + evalExpr + chain-lightning example

- Add 'when' column to scene table (generateSceneHtml + parser)
- Add evalExpr() for general-purpose variable evaluation (preserves any type)
- Fix --role regex to not capture --flags as IDs (negative lookahead)
- Chain-lightning example: recursive beam using when + --role + cast().without()
- Remove debug logs