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.

crates/components/src/expansion_valve.rs

@@ -222,6 +222,36 @@ impl ExpansionValve<Disconnected> {
     pub fn is_effectively_off(&self) -> bool {
         is_effectively_off_impl(self.operational_state, self.opening)
     }
+    /// Connects the expansion valve to inlet and outlet ports.
+    ///
+    /// This consumes the disconnected valve and returns a connected one,
+    /// transitioning the state at compile time.
+    pub fn connect(
+        self,
+        inlet: Port<Disconnected>,
+        outlet: Port<Disconnected>,
+    ) -> Result<ExpansionValve<Connected>, ComponentError> {
+        let (p_in, _) = self
+            .port_inlet
+            .connect(inlet)
+            .map_err(|e| ComponentError::InvalidState(e.to_string()))?;
+        let (p_out, _) = self
+            .port_outlet
+            .connect(outlet)
+            .map_err(|e| ComponentError::InvalidState(e.to_string()))?;
+
+        Ok(ExpansionValve {
+            port_inlet: p_in,
+            port_outlet: p_out,
+            calib: self.calib,
+            calib_indices: self.calib_indices,
+            operational_state: self.operational_state,
+            opening: self.opening,
+            fluid_id: self.fluid_id,
+            circuit_id: self.circuit_id,
+            _state: PhantomData,
+        })
+    }
 }
 
 /// Phase region at a thermodynamic state point.
@@ -603,6 +633,18 @@ impl Component for ExpansionValve<Connected> {
         2
     }
 
+    fn port_mass_flows(&self, state: &SystemState) -> Result<Vec<entropyk_core::MassFlow>, ComponentError> {
+        if state.len() < MIN_STATE_DIMENSIONS {
+            return Err(ComponentError::InvalidStateDimensions {
+                expected: MIN_STATE_DIMENSIONS,
+                actual: state.len(),
+            });
+        }
+        let m_in = entropyk_core::MassFlow::from_kg_per_s(state[0]);
+        let m_out = entropyk_core::MassFlow::from_kg_per_s(-state[1]); // Negative because it's leaving
+        Ok(vec![m_in, m_out])
+    }
+
     fn get_ports(&self) -> &[ConnectedPort] {
         &[]
     }