[Bugfix] Shade each XR eye from its own position

[miogds]

Summary

In XR every shading pass used the head-centre camera position for its view vector, so both eyes were lit as if seen from between them. Specular highlights and Fresnel then carry no stereo disparity: up close they read as painted onto the surface instead of sitting at the depth of a reflection. Each eye is now shaded from its own position.

Checked on an Apple Vision Pro through the editor's Mac-rendered preview (see Verification).

Cause

UntoldEngineXR.syncStreamingCameraPosition sets CameraComponent.localPosition to the headset centre once a frame (setXRCameraWorldPosition). That is right for streaming, where 3 cm do not matter. But the light pass and the other shading passes read the same value through SceneRootTransform.effectiveCameraPosition(cameraComponent.localPosition), while renderXR only updated each eye's viewSpace.

Changes

• renderXR stores the position of the eye it draws, taken from that eye's view matrix (eyePosition(fromViewMatrix:)), in renderInfo.xrEyeCameraPosition. The non-XR draw(in:) clears it.
• shadingCameraPosition(_:) returns that eye position, or the camera's own position outside XR, folded through the scene root as before. It replaces the 17 shading reads: the light pass, the model and batch uniforms of the geometry, shadow-free, transparency and debug passes, the sky, and render-extension surfaces.
• Unchanged on purpose:
  • Streaming, LOD, octree and culling keep the head centre.
  • Gaussian splats keep it too: their colour is evaluated once per frame in the preprocess shared by both eyes, and doing that per eye would double its cost.
• Outside XR nothing changes: the value is nil there, and the helper returns exactly what the passes used before.

Verification

• New ShadingCameraPositionTests (4): eye position from a rotated, translated view matrix; camera position outside XR; the eye, not the head centre, in XR; the scene root applied to the eye position.

• swift test --filter UntoldEngineTests on the rebased branch: 1,673 passed, 2 skipped (the external render extension package test run from a copy named UntoldEngine, where it passes). Strict-concurrency build: 0 warnings. xcodebuild -scheme UntoldEngine -destination 'generic/platform=visionOS Simulator': build succeeded.

• xcodebuild -scheme UntoldEngine -destination 'generic/platform=visionOS Simulator': build succeeded.

• No new SwiftFormat findings in the changed files.

• Checked on an Apple Vision Pro (the editor's preview, the Mac rendering both eyes), on the glass and paint of parked cars at arm's length with an HDR environment and a directional light, closing each eye in turn and switching the eye position off and on in the same build. With the head-centre position the reflection looks painted onto the glass; with each eye's own position it sits at its depth, and at some angles the two eyes see it slightly differently, as on real glass. Nothing else differed between the two runs.