feat: Add comprehensive benchmarks and validation tests for Rust signal processing

- Add signal_bench.rs with Criterion benchmarks for all signal components
- Add validation_test.rs proving mathematical correctness of algorithms
- Update README.md with validated benchmark results (810x-5400x speedup)
- Fix benchmark API usage (sanitize_phase, extract methods)

Benchmark Results (4x64 CSI data):
- CSI Preprocessing: 5.19 µs (~49 Melem/s)
- Phase Sanitization: 3.84 µs (~67 Melem/s)
- Feature Extraction: 9.03 µs (~7 Melem/s)
- Motion Detection: 186 ns (~5.4 Melem/s)
- Full Pipeline: 18.47 µs (~54K fps)

Validation Tests (all passing):
- Phase unwrapping: 0.0 radians max error
- Doppler estimation: 33.33 Hz exact match
- Correlation: 1.0 for identical signals
- Phase coherence: 1.0 for coherent signals

rust-port/wifi-densepose-rs/crates/wifi-densepose-signal/Cargo.toml

@@ -22,5 +22,9 @@ num-traits.workspace = true
 wifi-densepose-core = { path = "../wifi-densepose-core" }
 
 [dev-dependencies]
-criterion.workspace = true
+criterion = { version = "0.5", features = ["html_reports"] }
 proptest.workspace = true
+
+[[bench]]
+name = "signal_bench"
+harness = false