Add wifi-densepose-pointcloud: real-time dense point cloud from camera + WiFi CSI

New crate with 5 modules:
- depth: monocular depth estimation + 3D backprojection (ONNX-ready, synthetic fallback)
- pointcloud: Point3D/ColorPoint types, PLY export, Gaussian splat conversion
- fusion: WiFi occupancy volume → point cloud + multi-modal voxel fusion
- stream: HTTP + Three.js viewer server (Axum, port 9880)
- main: CLI with serve/capture/demo subcommands

Demo output: 271 WiFi points + 19,200 depth points → 4,886 fused → 1,718 Gaussian splats.
Serves interactive 3D viewer at http://localhost:9880 with Three.js orbit controls.

ADR-SYS-0021 documents the architecture for camera + WiFi CSI dense point cloud pipeline.

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

.gitignore

@@ -250,3 +250,5 @@ v1/src/sensing/mac_wifi
 # Local build scripts
 firmware/esp32-csi-node/build_firmware.batdata/
 models/
+demo_pointcloud.ply
+demo_splats.json