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/python/README.md

@@ -69,6 +69,20 @@ except entropyk.SolverError as e:
     print(f"Solver failed: {e}")
 ```
 
+## Recompiling after Rust Changes
+
+Because the Python bindings rely on the Rust source code (`crates/components`, `crates/solver`, etc.), you **must recompile the Python package** if you modify the underlying Rust physics engine.
+
+To recompile the bindings manually, simply use Maturin from the `bindings/python` directory with your virtual environment activated:
+
+```bash
+cd bindings/python
+source .venv/bin/activate
+maturin develop --release
+```
+
+*Note: If you added a new structural field in Rust (e.g. adding a `size` parameter to a Component struct), make sure to also update the Python wrapper class in `bindings/python/src/` so the macro `#[pyclass]` reflects the new shape before recompiling. You can use the `/update-python-bindings` agent workflow to do this automatically.*
+
 ## API Reference
 
 ### Physical Types