feat: implement mass balance validation for Story 7.1

- Added port_mass_flows to Component trait and implements for core components.
- Added System::check_mass_balance and integrated it into the solver.
- Restored connect methods for ExpansionValve, Compressor, and Pipe to fix integration tests.
- Updated Python and C bindings for validation errors.
- Updated sprint status and story documentation.

bindings/python/src/components.rs

@@ -91,6 +91,7 @@ impl std::fmt::Debug for SimpleAdapter {
 ///     )
 #[pyclass(name = "Compressor", module = "entropyk")]
 #[derive(Clone)]
+#[allow(dead_code)] // Fields reserved until SimpleAdapter type-state migration
 pub struct PyCompressor {
     pub(crate) coefficients: entropyk::Ahri540Coefficients,
     pub(crate) speed_rpm: f64,
@@ -381,6 +382,7 @@ impl PyExpansionValve {
 ///                 density=1140.0, viscosity=0.0002)
 #[pyclass(name = "Pipe", module = "entropyk")]
 #[derive(Clone)]
+#[allow(dead_code)] // Fields reserved until SimpleAdapter type-state migration
 pub struct PyPipe {
     pub(crate) length: f64,
     pub(crate) diameter: f64,