Full implementation of step() method with thermal-electrical coupling:
- Chamber temperature first-order response to setpoint
- Case temperature with self-heating via thermal calculations
- Junction temperature from θ_jc thermal resistance
- Electrical state from temperature-dependent DUT model
- Default LDO model when none provided
Define PhysicsEngine class with stub methods for thermal-electrical
simulation. Methods return placeholder values; full implementation
will be added in Sprint 3.