feat(rust): Add workspace deps, tests, and refine training modules

- Cargo.toml: Add wifi-densepose-train to workspace members; add
  petgraph, ndarray-npy, walkdir, sha2, csv, indicatif, clap to
  workspace dependencies
- error.rs: Slim down to focused error types (TrainError, DatasetError)
- lib.rs: Wire up all module re-exports correctly
- losses.rs: Add generate_gaussian_heatmaps implementation
- tests/test_config.rs: Deterministic config roundtrip and validation tests

https://claude.ai/code/session_01BSBAQJ34SLkiJy4A8SoiL4

rust-port/wifi-densepose-rs/crates/wifi-densepose-train/src/lib.rs

@@ -52,9 +52,9 @@ pub mod subcarrier;
 pub mod trainer;
 
 // Convenient re-exports at the crate root.
-pub use config::TrainingConfig;
-pub use dataset::{CsiDataset, CsiSample, DataLoader, MmFiDataset, SyntheticCsiDataset, SyntheticConfig};
-pub use error::{ConfigError, DatasetError, SubcarrierError, TrainError, TrainResult};
+pub use config::{ConfigError, TrainingConfig};
+pub use dataset::{CsiDataset, CsiSample, DataLoader, DatasetError, MmFiDataset, SyntheticCsiDataset, SyntheticConfig};
+pub use error::{TrainError, TrainResult};
 pub use subcarrier::{compute_interp_weights, interpolate_subcarriers, select_subcarriers_by_variance};
 
 /// Crate version string.