[Apple 2/5] Build the immersive visionOS experience and shared RealityKit viewer

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Goal

Recreate the interactive organ viewer in RealityKit, with visionOS delivered as a genuinely immersive spatial experience rather than a 2D screen in space.

Scope

• Build a shared RealityKit scene for organ loading, camera/orbit behavior, animation, selection, and hotspot picking.
• On visionOS, provide an ImmersiveSpace that places anatomy at convincing real-world scale.
• Support direct spatial manipulation and gaze/hand-driven selection using native interaction APIs.
• Provide window or volume UI only for controls and transition into/out of immersion.
• Adapt presentation and input for touch on iOS, pointer/keyboard on macOS, and focus/remote on tvOS.
• Preserve quiz, compare, autorotate, and structure-selection behavior where appropriate.

Acceptance criteria

• Vision Pro launches a full immersive anatomy scene with correct stereo depth and scale.
• The immersive scene is not implemented as a WebView or a flat full-screen panel.
• A person can enter and leave immersion safely and predictably.
• Native spatial input can select anatomy hotspots.
• The same scene/domain layer powers iOS, macOS, and tvOS presentations.
• Frame updates follow the platform display cadence and avoid unnecessary per-frame state commits.

Sequence

This is step 2 of 5 and depends on #18.

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First immersive vertical slice implemented on local branch codex/apple-platforms.

The visionOS app now declares a real ImmersiveSpace with mixed/full immersion styles. It renders the same RealityKit entity scene used by the windowed Apple targets, places content in meter-based immersive coordinates, supports native entity-targeted spatial selection, and provides explicit enter/leave controls. There is no WebView and no application-driven per-frame state loop.

The visionOS Simulator target compiles successfully. A lightweight procedural specimen currently proves the scene/input architecture; optimized USD assets, direct manipulation, full viewer parity, and runtime profiling remain tracked by #20-#22.