Add forceMonoscopic option for WebXR

[AmrKhamis1]

Related issue: #33177

Description

Adds optional forceMonoscopic flag to WebXRManager for single-viewpoint content. When enabled, both eyes use the left eye's view position, fixing projection misalignment on the right eye for content authored from a single viewpoint.

• renderer.xr.forceMonoscopic = true enables monoscopic mode
• Default remains stereo (backward compatible)

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	359.97 85.54	360.12 85.57	+148 B +32 B
WebGPU	630.72 175.04	630.72 175.04	+0 B +0 B
WebGPU Nodes	628.84 174.75	628.84 174.75	+0 B +0 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	492.17 120.12	492.32 120.16	+148 B +36 B
WebGPU	703.46 189.94	703.46 189.94	+0 B +0 B
WebGPU Nodes	652.69 177.33	652.69 177.33	+0 B +0 B

[Mugen87]

@cabanier Does this change look good to you?