feat(lib): Finalize async Experiment ABC — define interface contract and document public API

Goal

Finalize the asynchronous Abstract Base Class (ABC) for Experiment in plesty-lib. Experiments orchestrate one or more Devices to execute a measurement routine.

Requirements

• Define the public API: setup(), run(), teardown(), and data output contract
• Must compose cleanly with the Device ABC — an Experiment receives Device instances, not raw hardware
• All async-capable methods must be async
• Full type annotations and Mypy compliance
• Docstrings and a minimal runnable example in the module docs

Definition of Done

• ABC is merged and published
• The demo experiment in hub/experiments/demo successfully subclasses it
• plesty check passes on plesty-lib