Spatial video recording (MV-HEVC for Vision Pro)

Requested by @momentsintimeusa in #206.

What the user wants

Record spatial video (stereo MV-HEVC, playable in 3D on Apple Vision Pro) from the camera device, triggered and configured from the remote like any other video mode.

Platform facts

• Spatial video capture is available to third-party apps since iOS 17.2 (iPhone 15 Pro and later, wide + ultra-wide pair) via AVCaptureMovieFileOutput with isSpatialVideoCaptureEnabled, and since iOS 18 via AVCaptureVideoDataOutput + AVAssetWriter with the MV-HEVC multiview compression properties.
• Only formats with isSpatialVideoCaptureSupported qualify; supported formats are 1080p/30 (and a limited set on newer devices). Lens switching, zoom beyond the paired lenses, and non-16:9 crops are unavailable while spatial capture is on.
• Output is a .mov with two MV-HEVC layers; Photos tags it as Spatial automatically.

Scope sketch

• Camera: a spatial video mode in CaptureEngine/RecordingPipeline, gated on device support, that picks a spatial-capable format and locks out incompatible controls (lens switch, crop) while recording.
• Wire: capability flag supports_spatial_video + a VideoMode/resolution entry in FlatBufferSchemas.fbs so the remote only offers it to a capable camera (Unknown = 0 convention).
• Remote: mode picker entry; disable the controls the mode forbids.
• Transfer to remote / multicam: MV-HEVC files are larger; check the monitor-side receive path handles the size.

Open questions

• Is AVAssetWriter-based spatial capture (iOS 18) sufficient, or do we need the AVCaptureMovieFileOutput path for iOS 17.2 devices? Minimum target today is iOS 15, so this is feature-gated either way.
• Monetization: part of "enable video" IAP or a new tier?

Start by reading CaptureEngine and RecordingPipeline to determine how video modes and device formats are selected. Then inspect FlatBufferSchemas.fbs, the remote mode picker, and the monitor-side receive path for capability negotiation and file handling. Done means a supported camera can record spatial MV-HEVC, the remote exposes it only when valid, incompatible controls are disabled, and transfer works.