scd30: add CO2 sensor driver

[NetForces]

Closes #880.

Summary

• add an I2C driver for the Sensirion SCD30 CO2, temperature, and humidity sensor
• implement the drivers.Sensor API with cached fixed-point measurements
• support measurement interval and automatic self-calibration configuration, ambient-pressure compensation, data-ready checks, and continuous measurement start/stop
• validate Sensirion CRCs and use fixed-size device buffers
• add protocol tests, a Pico example, and smoke-test coverage

The protocol implementation this was extracted from has been exercised with an SCD30 on an M5Stack CoreInk using a clock-stretching-capable software I2C bus.

Validation

• make fmt-check
• make unit-test
• go vet ./scd30
• TinyGo v0.41.1 Pico build for examples/scd30
• make smoke-test