Replace Python numerical kernels with native C execution

app/simulation/core/equations.py

@@ -11,15 +11,14 @@ EquationRelation = Literal["equal", "sumToZero", "constitutive", "state"]
 
 
 @dataclass(frozen=True, slots=True)
-class EquationResidual:
-    """One executable scalar equation in the pressure-flow subsystem."""
+class EquationDefinition:
+    """One declarative equation in the compiled model interface."""
 
     id: str
     owner: EquationOwner
     owner_id: str
     relation: EquationRelation
     variables: tuple[str, ...]
-    value: float
     role: VariableRole | None = None
 
     def as_definition_dict(self) -> dict[str, object]:
@@ -33,4 +32,4 @@ class EquationResidual:
         }
 
     def as_interface_dict(self) -> dict[str, object]:
-        return {**self.as_definition_dict(), "residual": self.value}
+        return self.as_definition_dict()