Support native metro node #18

Parent: #4

Add renderer/native support for Elementary's metro node.

Docs: https://www.elementary.audio/docs/reference/metro

Scope:

• Confirm required renderer/runtime support for metro scheduling.
• Implement native bridge/runtime handling on iOS and Android as needed.
• Verify tempo/interval updates via render instructions and setProperty where applicable.
• Add example or test coverage proving el.metro(...) emits expected trigger behavior.

Acceptance criteria:

• el.metro can drive a simple graph in React Native without native/runtime errors.
• Interval/tempo changes behave consistently with Elementary docs.
• Works on both iOS and Android, or documents any platform limitation.