Add SwiftUI AR capture demo (ARCaptureManager + FrameRecorder + UI)

[EthanForAi]

Motivation

• Provide a minimal, runnable 6DoF data-capture demo using SwiftUI and ARKit to sample camera frames, optional scene depth, and camera transforms for later processing.
• Ship a self-contained example that writes per-session captures and a manifest describing images, timestamps, transforms, and depth data.

Description

• Add a SwiftUI app entry ARCaptureDemoApp and a ContentView that hosts an ARSessionView and simple Start/Stop capture controls connected to ARCaptureManager.
• Implement ARCaptureManager to manage an ARSession, handle session lifecycle and tracking state, and forward frames to FrameRecorder while publishing UI state via @Published properties.
• Implement FrameRecorder to sample frames at frameInterval, write JPEG images and raw depth bytes to Documents/captures/session_xxx/, and emit a metadata.json using CaptureRecord and CaptureSessionManifest models.
• Add ARSessionView as a UIViewRepresentable wrapper for ARSCNView, plus a README with setup notes and required Info.plist privacy keys.

Testing

• No automated tests were added or run for this change.

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Codex Task