Add world sensing

Build a complete sensing behavior: evidence enters the app, becomes current state, and produces an observable reaction.

1. Write the sensor contract

Name the physical or scene signal, cadence, observable reaction, data freshness, permissions, target runtime, and behavior for unsupported, denied, warming-up, empty, stale, and failed states.

Read references/branches.md, select the narrowest branch that supplies the signal, and read only that branch's manual and example.

Complete this step when one named signal maps to one reaction and every absence state maps to an explicit application state.

2. Prove the selected branch