Add physics engine stub

Define PhysicsEngine class with stub methods for thermal-electrical
simulation. Methods return placeholder values; full implementation
will be added in Sprint 3.

src/py_dvt_ate/simulation/physics/engine.py

@@ -0,0 +1,136 @@
+"""Physics engine for coupled thermal-electrical simulation.
+
+The physics engine maintains the simulation state and advances it
+in discrete time steps, modelling the thermal and electrical coupling
+between the DUT and its environment.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from py_dvt_ate.simulation.physics.models import ElectricalState, ThermalState
+
+if TYPE_CHECKING:
+    from py_dvt_ate.simulation.physics.models.base import DUTModel
+
+
+class PhysicsEngine:
+    """Coupled thermal-electrical physics simulation.
+
+    Runs at a fixed timestep, updating thermal and electrical state
+    based on the DUT model and environmental conditions.
+
+    Attributes:
+        dt: Simulation timestep in seconds.
+    """
+
+    def __init__(
+        self,
+        update_rate_hz: float = 100.0,
+        dut_model: DUTModel | None = None,
+    ) -> None:
+        """Initialise the physics engine.
+
+        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.
+        """
+        self.dt = 1.0 / update_rate_hz
+        self._dut = dut_model
+
+        # Thermal parameters (to be configured in Sprint 3)
+        self._tau_chamber = 30.0  # seconds
+        self._tau_case = 5.0  # seconds
+        self._theta_jc = 15.0  # degC/W
+        self._theta_ca = 5.0  # degC/W
+
+        # State variables
+        self._t_setpoint = 25.0
+        self._t_chamber = 25.0
+        self._t_case = 25.0
+        self._v_in = 0.0
+        self._i_load = 0.0
+        self._output_enabled = False
+        self._sim_time = 0.0
+
+    def step(self) -> None:
+        """Advance simulation by one timestep.
+
+        Updates thermal and electrical state based on current conditions.
+        Full implementation in Sprint 3.
+        """
+        # Stub: just advance time
+        self._sim_time += self.dt
+
+    def get_thermal_state(self) -> ThermalState:
+        """Get current thermal state snapshot.
+
+        Returns:
+            Immutable ThermalState with current temperatures.
+        """
+        # Stub: return current state values
+        return ThermalState(
+            chamber_temperature=self._t_chamber,
+            case_temperature=self._t_case,
+            junction_temperature=self._t_case,  # Stub: junction = case
+            timestamp=self._sim_time,
+        )
+
+    def get_electrical_state(self) -> ElectricalState:
+        """Get current electrical state snapshot.
+
+        Returns:
+            Immutable ElectricalState with current electrical values.
+        """
+        # Stub: return placeholder values
+        return ElectricalState(
+            input_voltage=self._v_in,
+            output_voltage=0.0,  # Stub: will be calculated from DUT model
+            load_current=self._i_load if self._output_enabled else 0.0,
+            quiescent_current=0.0,  # Stub: will be calculated from DUT model
+            power_dissipation=0.0,  # Stub: will be calculated from DUT model
+        )
+
+    def set_chamber_setpoint(self, temperature: float) -> None:
+        """Set chamber target temperature.
+
+        Args:
+            temperature: Target temperature in degrees Celsius.
+        """
+        self._t_setpoint = temperature
+
+    def set_input_voltage(self, voltage: float) -> None:
+        """Set DUT input voltage.
+
+        Args:
+            voltage: Input voltage in volts.
+        """
+        self._v_in = voltage
+
+    def set_load_current(self, current: float) -> None:
+        """Set DUT load current.
+
+        Args:
+            current: Load current in amps.
+        """
+        self._i_load = current
+
+    def set_output_enabled(self, enabled: bool) -> None:
+        """Enable or disable DUT power.
+
+        Args:
+            enabled: True to enable output, False to disable.
+        """
+        self._output_enabled = enabled
+
+    @property
+    def simulation_time(self) -> float:
+        """Get current simulation time in seconds."""
+        return self._sim_time
+
+    @property
+    def is_output_enabled(self) -> bool:
+        """Check if DUT output is enabled."""
+        return self._output_enabled