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.

Cargo.toml

@@ -7,6 +7,8 @@ members = [
     "demo",             # Demo/test project (user experiments)
     "crates/solver",
     "bindings/python",  # Python bindings (PyO3)
+    "bindings/c",       # C FFI bindings (cbindgen)
+    "bindings/wasm",    # WebAssembly bindings (wasm-bindgen)
 ]
 resolver = "2"