Removed mathematical singularity in HeatExchanger models (q_hot - q_cold = 0 was redundant) causing them to incorrectly request 3 equations without internal variables. Fixed ScrewEconomizerCompressor internal_state_len to perfectly align with the solver dimensions.
- 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.
- Fixed Critical issue: Wired up _state to the underlying HeatExchanger boundary conditions so the Newton-Raphson solver actually sees numerical gradients.
- Fixed Critical issue: Bubble up FluidBackend errors via ComponentError::CalculationFailed instead of silently swallowing backend evaluation failures.
- Fixed Medium issue: Connected condenser_with_backend into the eurovent.rs system architecture so the demo solves instead of just printing output.
- Fixed Medium issue: Removed heavy FluidId clones inside query loop.
- Fixed Low issue: Added physical validations to HxSideConditions.
