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/compressor.rs

@@ -652,6 +652,39 @@ impl Compressor<Disconnected> {
     pub fn set_operational_state(&mut self, state: OperationalState) {
         self.operational_state = state;
     }
+    /// Connects the compressor to suction and discharge ports.
+    ///
+    /// This consumes the disconnected compressor and returns a connected one,
+    /// transitioning the state at compile time.
+    pub fn connect(
+        self,
+        suction: Port<Disconnected>,
+        discharge: Port<Disconnected>,
+    ) -> Result<Compressor<Connected>, ComponentError> {
+        let (p_suction, _) = self
+            .port_suction
+            .connect(suction)
+            .map_err(|e| ComponentError::InvalidState(e.to_string()))?;
+        let (p_discharge, _) = self
+            .port_discharge
+            .connect(discharge)
+            .map_err(|e| ComponentError::InvalidState(e.to_string()))?;
+
+        Ok(Compressor {
+            model: self.model,
+            port_suction: p_suction,
+            port_discharge: p_discharge,
+            speed_rpm: self.speed_rpm,
+            displacement_m3_per_rev: self.displacement_m3_per_rev,
+            mechanical_efficiency: self.mechanical_efficiency,
+            calib: self.calib,
+            calib_indices: self.calib_indices,
+            fluid_id: self.fluid_id,
+            circuit_id: self.circuit_id,
+            operational_state: self.operational_state,
+            _state: PhantomData,
+        })
+    }
 }
 
 impl Compressor<Connected> {
@@ -1217,6 +1250,22 @@ impl Component for Compressor<Connected> {
         2 // Mass flow residual and energy residual
     }
 
+    fn port_mass_flows(&self, state: &SystemState) -> Result<Vec<entropyk_core::MassFlow>, ComponentError> {
+        if state.len() < 4 {
+            return Err(ComponentError::InvalidStateDimensions {
+                expected: 4,
+                actual: state.len(),
+            });
+        }
+        let m = entropyk_core::MassFlow::from_kg_per_s(state[0]);
+        // Suction (inlet), Discharge (outlet), Oil (no flow modeled yet)
+        Ok(vec![
+            m, 
+            entropyk_core::MassFlow::from_kg_per_s(-m.to_kg_per_s()), 
+            entropyk_core::MassFlow::from_kg_per_s(0.0)
+        ])
+    }
+
     fn get_ports(&self) -> &[ConnectedPort] {
         // NOTE: This returns an empty slice due to lifetime constraints.
         // Use `get_ports_slice()` method on Compressor<Connected> for actual port access.