Phase 2 — WebGPU XR projection layer + render target provider

Tracking issue: #18640 · Epic: #18635 · Follows Phase 0 (#18645) and Phase 1 (#18650).

Goal

Implement the core WebGPU XR rendering path: render the stereo scene into an XRProjectionLayer's per‑view GPUTextures via XRGPUBinding, so a WebGPU XR session on Quest reaches IN_XR and actually renders. Phase 1 enters the session but stays blank because no layer is attached and no RTT provider produces textures — this PR closes that gap.

WebGL2 XR remains byte‑for‑byte unchanged. Everything new is gated behind isWebGPU / binding‑type branches, is @internal, and is kept out of the public barrels. Zero net public‑API additions.

What's in this PR (two commits for reviewability, one mergeable unit)

Commit 1 — WebGPU RTT providers (additive, inert until commit 2):

• XR/webXRWebGPURenderTargetTextureProvider.ts — abstract @internal base extending the API‑agnostic WebXRLayerRenderTargetTextureProvider. Wraps per‑view color/depth GPUTextures via WebGPUEngine.wrapWebGPUTexture, sets the correct Babylon depth format on the wrapped depth texture, repoints per‑frame via updateWrappedWebGPUTexture (rebuilds only on size change), and builds RTTs via _createRenderTargetTextureInternal.
• XR/features/Layers/WebXRWebGPUCompositionLayer.ts — WebGPU composition provider + wrapper mirroring the WebGL provider over XRGPUSubImage (dimensions from colorTexture.width/height), size‑keyed cache with repoint fast‑path and viewport normalization.
• XR/features/Layers/WebXRWebGPUProjectionLayer.ts — WebGPU projection wrapper/provider using getViewSubImage, plus the default XRGPUProjectionLayerInit factory.
• Unit tests (test/unit/XR/webXRWebGPUProjectionLayer.test.ts).

Commit 2 — WebXRLayers activation:

• XR/features/WebXRLayers.pure.ts — branches attach() / isCompatible() on engine.isWebGPU to create an XRGPUBinding projection layer (via the Phase 1 graphics binding) and instantiate the WebGPU wrapper. The WebGL branch is only bypassed when isWebGPU and an XRGPUBinding is present.

Key implementation notes / fork resolutions

• Depth‑format gap (fixed in the provider): wrapWebGPUTexture doesn't set the Babylon InternalTexture.format; the WebGPU depth attachment path reads it. The provider sets the mapped Babylon depth format on the wrapped depth texture. wrapWebGPUTexture is left untouched (minimal general‑purpose primitive).
• Per‑frame texture rotation: updateWrappedWebGPUTexture fast path preserves RTT/InternalTexture identity (held by Camera.outputRenderTarget) and bumps uniqueId to invalidate bind‑group caches; full re‑wrap only on size change (the update guard requires matching dims).
• External‑texture targeting: relies on the existing Camera.outputRenderTarget → bindFramebuffer → _startRenderTargetRenderPass path. The swapchain (getCurrentTexture) is only used when no render target is bound, so it is untouched during XR.
• MSAA (ready-to-enable, deferred): ships with samples=1 for Phase 2. Babylon‑managed MSAA resolve into the external single‑sample color texture already works via the existing getMSAATexture/resolveTarget path and needs only RTT samples>1 (no engine work). Deliberately deferred to isolate the first hardware validation — enabling it will be a small fast-follow after RaananW confirms the base stereo render works on Quest.
• Multiview: single‑view only (two eyes / two sub‑images via getViewSubImage); multiview=false for WebGPU. Multiview is deferred (Phase 4+).

Scope boundaries

• Phase 3 ([WebGPU-XR][Phase 3] WebXR camera projection matrix / NDC handling for WebGPU #18639): camera projection matrices, NDC / depth [0,1] range — NOT touched here.
• Phase 4: other layer types (quad/cylinder/equirect/cube), ADT/lens‑flare over WebGPU, multiview — excluded.

Testing

• tsc -b (core): clean.
• Unit tests: 7/7 new pass; full XR unit suite 266/266 pass.
• npm run format:check, npm run lint:check (incl. 16 tree‑shaking checks + side‑effects‑sync): all green.
• ⚠️ Actual stereo WebGPU‑XR rendering can only be validated on Quest hardware by @RaananW — no WebGPU‑XR browser is available in CI/dev here. Please smoke‑test the built snapshot on device.