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/wasm/src/errors.rs

@@ -0,0 +1,10 @@
+//! Error handling for WASM bindings.
+//!
+//! Maps errors to JavaScript exceptions with human-readable messages.
+
+use wasm_bindgen::JsValue;
+
+/// Convert a Result to a Result with JsValue error.
+pub fn result_to_js<T, E: std::fmt::Display>(result: Result<T, E>) -> Result<T, JsValue> {
+    result.map_err(|e| js_sys::Error::new(&e.to_string()).into())
+}