feat(pointcloud): add Rust Depth Anything backend

[ruvnet]

Summary

• accept ADR-271 for an optional Rust-owned depth-anything.cpp backend
• dynamically load C ABI v4 without imposing C++ on ordinary RuView builds
• preserve calibrated dimensions/intrinsics, metric status, confidence, and RGB alignment through point-cloud fusion
• retain auto fallback to MiDaS and the deterministic heuristic, while explicit Depth Anything mode fails closed
• enforce declared model licensing and optional SHA-256 verification; no weights are bundled
• add ruview-pointcloud depth-check for ABI-only and real inference validation

Validation

• cargo test -p wifi-densepose-pointcloud — 33 passed
• cargo clippy -p wifi-densepose-pointcloud --all-targets -- -D warnings
• cargo build --release -p wifi-densepose-pointcloud
• real Windows/MSVC build of upstream ABI v4: symbol probe passed
• real Apache-2.0 DA2 metric Hypersim small Q4_K inference passed at 518x518, metric output 1.4951–1.7172 m
• validated model SHA-256: decd99f5c756b564aae5ca1a1612f896e7f76889060e1d25ba610549bbc39b52

Current native limitations

• model weights remain a separately provisioned runtime asset
• the upstream Windows DLL currently needs CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS=ON
• a validated upstream DA2 destructor access violation requires retaining the Windows process-singleton context until process teardown
• the upstream path-only image ABI requires a create-new, automatically removed PPM bridge; an in-memory RGB ABI is the intended successor
• target camera/hardware validation remains pending receipt of hardware

Architecture

See docs/adr/ADR-271-depth-anything-rust-camera-depth-backend.md.