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
Temperature-dependent LDO voltage regulator model with:
- Output voltage tempco (ppm/°C)
- Quiescent current tempco
- Dropout voltage temperature dependence
- Power dissipation calculation (Vin-Vout)*Iload + Vin*Iq
- Dropout detection
Implements DUTModel protocol for physics engine integration.
Pure functions for first-order thermal response calculations:
- Temperature derivative and update using Euler integration
- Case temperature with self-heating via θ_ca
- Junction temperature calculation via θ_jc
- Steady-state junction temperature helper
Define PhysicsEngine class with stub methods for thermal-electrical
simulation. Methods return placeholder values; full implementation
will be added in Sprint 3.
