feat: ADR-021 vital sign detection + RVF container format (closes #45)

Implement WiFi CSI-based vital sign detection and RVF model container:

- Pure-Rust radix-2 DIT FFT with Hann windowing and parabolic interpolation
- FIR bandpass filter (windowed-sinc, Hamming) for breathing (0.1-0.5 Hz)
  and heartbeat (0.8-2.0 Hz) band isolation
- VitalSignDetector with rolling buffers (30s breathing, 15s heartbeat)
- RVF binary container with 64-byte SegmentHeader, CRC32 integrity,
  6 segment types (Vec, Manifest, Quant, Meta, Witness, Profile)
- RvfBuilder/RvfReader with file I/O and VitalSignConfig support
- Server integration: --benchmark, --load-rvf, --save-rvf CLI flags
- REST endpoint /api/v1/vital-signs and WebSocket vital_signs field
- 98 tests (32 unit + 16 RVF integration + 18 vital signs integration)
- Benchmark: 7,313 frames/sec (136μs/frame), 365x real-time at 20 Hz

Co-Authored-By: claude-flow <ruv@ruv.net>

rust-port/wifi-densepose-rs/crates/wifi-densepose-sensing-server/Cargo.toml

@@ -5,6 +5,10 @@ edition.workspace = true
 description = "Lightweight Axum server for WiFi sensing UI with RuVector signal processing"
 license.workspace = true
 
+[lib]
+name = "wifi_densepose_sensing_server"
+path = "src/lib.rs"
+
 [[bin]]
 name = "sensing-server"
 path = "src/main.rs"
@@ -29,3 +33,6 @@ chrono = { version = "0.4", features = ["serde"] }
 
 # CLI
 clap = { workspace = true }
+
+[dev-dependencies]
+tempfile = "3.10"