Implement physics engine stepping

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

src/py_dvt_ate/simulation/physics/engine.py

@@ -10,6 +10,11 @@ from __future__ import annotations
 from typing import TYPE_CHECKING
 
 from py_dvt_ate.simulation.physics.state import ElectricalState, ThermalState
+from py_dvt_ate.simulation.physics.thermal import (
+    calculate_junction_temperature,
+    update_case_temperature,
+    update_temperature,
+)
 
 if TYPE_CHECKING:
     from py_dvt_ate.simulation.physics.models.base import DUTModel
@@ -21,6 +26,12 @@ class PhysicsEngine:
     Runs at a fixed timestep, updating thermal and electrical state
     based on the DUT model and environmental conditions.
 
+    The simulation models:
+    - Chamber temperature approaching setpoint (first-order response)
+    - Case temperature driven by chamber and self-heating
+    - Junction temperature from case temperature and thermal resistance
+    - Electrical behaviour from the DUT model (temperature-dependent)
+
     Attributes:
         dt: Simulation timestep in seconds.
     """
@@ -35,12 +46,19 @@ class PhysicsEngine:
         Args:
             update_rate_hz: Simulation update rate in Hz. Defaults to 100.
             dut_model: DUT model to use for electrical calculations.
-                       If None, a default model will be used when implemented.
+                       If None, a default LDO model will be used.
         """
         self.dt = 1.0 / update_rate_hz
-        self._dut = dut_model
 
-        # Thermal parameters (to be configured in Sprint 3)
+        # Lazily import to avoid circular dependencies
+        if dut_model is None:
+            from py_dvt_ate.simulation.physics.models.ldo import LDOModel
+
+            self._dut: DUTModel = LDOModel()
+        else:
+            self._dut = dut_model
+
+        # Thermal parameters
         self._tau_chamber = 30.0  # seconds
         self._tau_case = 5.0  # seconds
         self._theta_jc = 15.0  # degC/W
@@ -59,9 +77,34 @@ class PhysicsEngine:
         """Advance simulation by one timestep.
 
         Updates thermal and electrical state based on current conditions.
-        Full implementation in Sprint 3.
+        The thermal-electrical coupling works as follows:
+        1. Calculate current power dissipation from DUT model
+        2. Update chamber temperature towards setpoint
+        3. Update case temperature including self-heating
+        4. Advance simulation time
         """
-        # Stub: just advance time
+        # Calculate power dissipation (uses current junction temperature estimate)
+        p_diss = self._calculate_power_dissipation()
+
+        # Update chamber temperature (first-order response to setpoint)
+        self._t_chamber = update_temperature(
+            current_temperature=self._t_chamber,
+            target_temperature=self._t_setpoint,
+            time_constant=self._tau_chamber,
+            dt=self.dt,
+        )
+
+        # Update case temperature (driven by chamber + self-heating)
+        self._t_case = update_case_temperature(
+            case_temperature=self._t_case,
+            ambient_temperature=self._t_chamber,
+            power_dissipation=p_diss,
+            time_constant=self._tau_case,
+            theta_ca=self._theta_ca,
+            dt=self.dt,
+        )
+
+        # Advance simulation time
         self._sim_time += self.dt
 
     def get_thermal_state(self) -> ThermalState:
@@ -70,11 +113,17 @@ class PhysicsEngine:
         Returns:
             Immutable ThermalState with current temperatures.
         """
-        # Stub: return current state values
+        p_diss = self._calculate_power_dissipation()
+        t_junction = calculate_junction_temperature(
+            case_temperature=self._t_case,
+            power_dissipation=p_diss,
+            theta_jc=self._theta_jc,
+        )
+
         return ThermalState(
             chamber_temperature=self._t_chamber,
             case_temperature=self._t_case,
-            junction_temperature=self._t_case,  # Stub: junction = case
+            junction_temperature=t_junction,
             timestamp=self._sim_time,
         )
 
@@ -84,13 +133,50 @@ class PhysicsEngine:
         Returns:
             Immutable ElectricalState with current electrical values.
         """
-        # Stub: return placeholder values
+        p_diss = self._calculate_power_dissipation()
+        t_junction = calculate_junction_temperature(
+            case_temperature=self._t_case,
+            power_dissipation=p_diss,
+            theta_jc=self._theta_jc,
+        )
+
+        if self._output_enabled:
+            v_out = self._dut.calculate_output_voltage(t_junction)
+            i_q = self._dut.calculate_quiescent_current(t_junction)
+            i_load = self._i_load
+        else:
+            v_out = 0.0
+            i_q = 0.0
+            i_load = 0.0
+
         return ElectricalState(
             input_voltage=self._v_in,
-            output_voltage=0.0,  # Stub: will be calculated from DUT model
+            output_voltage=v_out,
-            load_current=self._i_load if self._output_enabled else 0.0,
+            load_current=i_load,
-            quiescent_current=0.0,  # Stub: will be calculated from DUT model
+            quiescent_current=i_q,
-            power_dissipation=0.0,  # Stub: will be calculated from DUT model
+            power_dissipation=p_diss,
+        )
+
+    def _calculate_power_dissipation(self) -> float:
+        """Calculate current power dissipation.
+
+        Uses the current case temperature as an approximation for junction
+        temperature in the power calculation. The true junction temperature
+        depends on power dissipation, creating a feedback loop that is
+        resolved iteratively through the simulation steps.
+
+        Returns:
+            Power dissipation in watts.
+        """
+        if not self._output_enabled:
+            return 0.0
+
+        # Use case temperature as junction estimate for power calculation
+        # This avoids circular dependency in the calculation
+        return self._dut.calculate_power_dissipation(
+            input_voltage=self._v_in,
+            load_current=self._i_load,
+            junction_temperature=self._t_case,
         )
 
     def set_chamber_setpoint(self, temperature: float) -> None: