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/entropyk/src/error.rs

@@ -63,6 +63,15 @@ pub enum ThermoError {
     /// System was not finalized before an operation.
     #[error("System must be finalized before this operation")]
     NotFinalized,
+
+    /// Simulation validation error (e.g., mass/energy balance constraints violated)
+    #[error("Validation failed: mass error = {mass_error:.3e} kg/s, energy error = {energy_error:.3e} W")]
+    Validation {
+        /// Mass balance error in kg/s
+        mass_error: f64,
+        /// Energy balance error in W
+        energy_error: f64,
+    },
 }
 
 impl ThermoError {