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/c/Cargo.toml

@@ -0,0 +1,23 @@
+[package]
+name = "entropyk-c"
+description = "C FFI bindings for the Entropyk thermodynamic simulation library"
+version.workspace = true
+authors.workspace = true
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+
+[lib]
+name = "entropyk_ffi"
+crate-type = ["staticlib", "cdylib"]
+
+[dependencies]
+entropyk = { path = "../../crates/entropyk" }
+entropyk-core = { path = "../../crates/core" }
+entropyk-components = { path = "../../crates/components" }
+entropyk-solver = { path = "../../crates/solver" }
+entropyk-fluids = { path = "../../crates/fluids" }
+petgraph = "0.6"
+
+[build-dependencies]
+cbindgen = "0.28"