Rename models.py to state.py to avoid conflict with models/ directory

The models.py file conflicts with the models/ subdirectory when
importing. Renamed to state.py for clarity.

src/py_dvt_ate/simulation/physics/state.py

@@ -0,0 +1,48 @@
+"""Physics state dataclasses for thermal-electrical simulation.
+
+These immutable state snapshots represent the simulation state at a point in time.
+"""
+
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class ThermalState:
+    """Immutable thermal state snapshot.
+
+    Represents the thermal conditions of the DUT and its environment
+    at a specific point in simulation time.
+
+    Attributes:
+        chamber_temperature: Chamber air temperature in degrees Celsius.
+        case_temperature: DUT case/package temperature in degrees Celsius.
+        junction_temperature: DUT junction/die temperature in degrees Celsius.
+        timestamp: Simulation time in seconds since start.
+    """
+
+    chamber_temperature: float
+    case_temperature: float
+    junction_temperature: float
+    timestamp: float
+
+
+@dataclass(frozen=True)
+class ElectricalState:
+    """Immutable electrical state snapshot.
+
+    Represents the electrical conditions of the DUT at a specific
+    point in simulation time.
+
+    Attributes:
+        input_voltage: DUT input voltage in volts.
+        output_voltage: DUT output voltage in volts.
+        load_current: Load current drawn from DUT in amps.
+        quiescent_current: DUT quiescent/bias current in amps.
+        power_dissipation: Total power dissipated by DUT in watts.
+    """
+
+    input_voltage: float
+    output_voltage: float
+    load_current: float
+    quiescent_current: float
+    power_dissipation: float