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/Cargo.toml

@@ -11,6 +11,7 @@ members = [
     "crates/wifi-densepose-wasm",
     "crates/wifi-densepose-cli",
     "crates/wifi-densepose-mat",
+    "crates/wifi-densepose-train",
 ]
 
 [workspace.package]
@@ -73,6 +74,25 @@ getrandom = { version = "0.2", features = ["js"] }
 serialport = "4.3"
 pcap = "1.1"
 
+# Graph algorithms (for min-cut assignment in metrics)
+petgraph = "0.6"
+
+# Data loading
+ndarray-npy = "0.8"
+walkdir = "2.4"
+
+# Hashing (for proof)
+sha2 = "0.10"
+
+# CSV logging
+csv = "1.3"
+
+# Progress bars
+indicatif = "0.17"
+
+# CLI
+clap = { version = "4.4", features = ["derive"] }
+
 # Testing
 criterion = { version = "0.5", features = ["html_reports"] }
 proptest = "1.4"