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/lib.rs

@@ -100,6 +100,7 @@ pub use state_machine::{
     StateTransitionRecord,
 };
 
+use entropyk_core::MassFlow;
 use thiserror::Error;
 
 /// Errors that can occur during component operations.
@@ -543,6 +544,23 @@ pub trait Component {
         0
     }
 
+    /// Returns the mass flow vector associated with the component's ports.
+    /// 
+    /// The returned vector matches the order of ports returned by `get_ports()`.
+    /// Positive values indicate flow *into* the component, negative values flow *out*.
+    /// 
+    /// # Arguments
+    /// 
+    /// * `state` - The global system state vector
+    /// 
+    /// # Returns
+    /// 
+    /// * `Ok(Vec<MassFlow>)` containing the mass flows if calculation is supported
+    /// * `Err(ComponentError::NotImplemented)` by default
+    fn port_mass_flows(&self, _state: &SystemState) -> Result<Vec<MassFlow>, ComponentError> {
+        Err(ComponentError::CalculationFailed("Mass flow calculation not implemented for this component".to_string()))
+    }
+
     /// Injects control variable indices for calibration parameters into a component.
     ///
     /// Called by the solver (e.g. `System::finalize()`) after matching `BoundedVariable`s