Add a sensor

A sensor is a driver in rohi-hal managed by a generic Runner; the board crate supplies the bus, the firmware spawns one task. Drivers know nothing about networks or serialization. Background: Sensor HAL, Sensors.

1. Model the observation (portable, rohi-hal). Add the sensor to SensorKind and give it a typed update in MeasurementUpdate (units from units.rs: Celsius, Ppm, …). Measurements are grouped by the physical sensor; never merge readings of different sensors. Give the sensor a freshness policy (FreshnessPolicy::default_for) that outlasts its worst healthy cadence: a test in sensors/mod.rs enforces it.
2. Write the driver in crates/rohi-hal/src/sensors/<name>.rs and implement SensorDriver: init (probe and configure; called again to recover), sample (wait for and read the next observation, Ok(None) = no data yet), poll_interval_ms, optionally initial_delay_ms, plus KIND and WARMS_UP. Map the bus error into DriverError (code, and is_absent for “does not answer”, which makes the state NotDetected instead of Error).
3. Test on the host. In the driver module, drive the driver with MockI2c / a fake UART from the crate-internal testutil (crates/rohi-hal/src/testutil.rs), and the runner with a recording delay. Cover detection, a failing bus, recovery, warm-up and the exact decoding of datasheet examples. See Host testing.
4. Wire the bus (platform). If the sensor needs pins or a new peripheral, add them to the board module in rohi-hal-esp (see Add a board); drivers never name pins.
5. Spawn a task in firmwares/altruist-urban/src/tasks/sensors.rs: build the driver, wrap it in Runner::new(driver, reporter(SensorKind::X), Delay, RunnerConfig::DEFAULT) and runner.run().await; start it in main.rs with the other sensor tasks. I²C sensors share one bus through i2c_handle.
6. Expose it to the control plane if it should be visible: extend the mapping from the typed snapshot to the protobuf Sensor list in rohi-control and the canonical unit table in Control plane (see Add a control operation for schema rules).
7. Add a hardware example in examples/src/bin/ if useful, and record the validation in Hardware testing.
8. Update the docs (policy): the sensor’s page section in Sensors and any new limit in Resource limits.

Rules to respect: the runner owns the driver (one task per sensor); do not call step in a select unless you accept that a bus transaction may be cut short (cancellation); do not allocate.