Add native iOS foundation

[cseelus]

What changed

• establishes web/, ios/, shared/, and backend/ repository boundaries
• adds a native SwiftUI FIELD app preserving the prototype’s visual language
• connects ARKit/LiDAR scene depth, live microphone level, location, heading, motion, barometer, and relative altitude behind explicit scan actions
• keeps AI synthesis clearly labeled as a guided demo
• adds Codex cloud setup guidance plus web and macOS GitHub Actions checks

Why

This creates the first installable native slice for real iPhone 17 Pro testing while keeping routine development cloud-friendly and the web prototype independently deployable.

Validation

• cd web && npm test
• unsigned Xcode simulator build with Xcode 26.6
• git diff --check

Device handoff

Choose a personal team in Xcode, connect the iPhone, and run the FIELD scheme. LiDAR depth and the physical sensor path must be verified on device.