Add native cube render target support (re-enables 2 shadow-with-instances tests)

[bkaradzic-microsoft]

Paired engine PR: BabylonJS/Babylon.js#18561

What

Implements cube render targets on the native engine so ReflectionProbe and
point-light cube shadow maps render instead of crashing on the null WebGL
context. Pairs with the Babylon.js change that adds the JS-side overrides.

Changes

• Plugins/NativeEngine/Source/NativeEngine.cpp
  • InitializeTexture: optional isCube param → Texture::CreateCube.
  • CreateFrameBuffer: optional layer param so the color attachment targets
    a single cube face (bgfx::Attachment layer). The JS side creates one
    framebuffer per face and binds the matching one per face.
• Apps/Playground/Scripts/config.json: re-enables "Shadows with instances in
  left/right handed system" (previously excluded as crash/hang).

CI

The validation suite uses the published babylonjs npm, which does not yet
contain the paired JS overrides, so the re-enabled tests stay red until a
babylonjs release with the Babylon.js change ships and the dependency is
bumped here. Draft until then.

Verified locally

Built against a local babylon.max.js with the paired change: the two
re-enabled tests pass and there are no regressions in post-process / 2D
render-target tests.

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Related PRs & landing order

• Babylon.js (engine / TS): [Native] Add cube render target support to the native engine Babylon.js#18561
• BabylonNative (C++ engine + test re-enable): Add native cube render target support (re-enables 2 shadow-with-instances tests) #1750

Co-dependent; land in this order:

1. Babylon.js #18561 first — adds the cube render-target TS overrides; no WebGL behavior change.
2. A babylonjs npm release ships that TS change.
3. BabylonNative Add native cube render target support (re-enables 2 shadow-with-instances tests) #1750 last — bumps the bundled babylonjs and re-enables the 2 Shadows with instances (left/right handed) validation tests, which only pass once the paired JS is present in the bundled engine.

[bkaradzic-microsoft]

TODO at landing: bump the native PROTOCOL_VERSION (Plugins/NativeEngine/Source/NativeEngine.cpp, the StaticValue("PROTOCOL_VERSION", ...)) from 9 → 10 to pair with the JS-side bump in BabylonJS/Babylon.js#18561 (the cube-RT version gate from bghgary's review).

This must land in the same change that bumps the babylonjs pin to a release containing #18561's PROTOCOL_VERSION = 10. Bumping it standalone now would make every test here throw Protocol version mismatch at engine init against the currently-bundled 9.x, so it's deferred until the coordinated pin bump.