#[tool]Expand description
Link a participant state struct to its Config as a raw-bus tool (Api
defaults to () - tools stay raw-bus only).
The tool-shaped counterpart to [service]. Api and Config default to
() - tools stay raw-bus only (decided 2026-07-09), and a configless tool
can launch without PHOXAL_CONFIG. An explicit config = Type remains
required at launch unless that type itself accepts null (for example,
Option<T>).