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.

_bmad-output/implementation-artifacts/sprint-status.yaml

@@ -101,15 +101,15 @@ development_status:
   # Epic 6: Multi-Platform APIs
   epic-6: in-progress
   6-1-rust-native-api: done
-  6-2-python-bindings-pyo3: in-progress
-  6-3-c-ffi-bindings-cbindgen: ready-for-dev
-  6-4-webassembly-compilation: backlog
+  6-2-python-bindings-pyo3: done
+  6-3-c-ffi-bindings-cbindgen: done
+  6-4-webassembly-compilation: in-progress
   6-5-cli-for-batch-execution: backlog
   epic-6-retrospective: optional
 
   # Epic 7: Validation & Persistence
-  epic-7: backlog
-  7-1-mass-balance-validation: backlog
+  epic-7: in-progress
+  7-1-mass-balance-validation: review
   7-2-energy-balance-validation: backlog
   7-3-traceability-metadata: backlog
   7-4-debug-verbose-mode: backlog