NativeEngine: load single-file .dds/.ktx/.ktx2 cubemaps + spherical harmonics (re-enables 6 PBR tests)

[bkaradzic-microsoft]

NativeEngine: load single-file .dds/.ktx/.ktx2 cubemaps + compute spherical harmonics

What

Adds native support for loading a self-contained cubemap container (.dds / .ktx / .ktx2,
the format produced by CubeTexture.CreateFromPrefilteredData(...)) and re-enables six PBR
environment validation tests.

Plugins/NativeEngine/Source/NativeEngine.cpp:

• loadCubeTexture now accepts a single buffer (all six faces + mips in one container).
  ParseCubeImage runs it through bimg::imageParse (which decodes DDS/KTX/KTX2 cubemaps), and
  LoadCubeTextureFromContainer uploads sides 0–5 × mips.
• ComputeCubeSphericalPolynomial derives the diffuse-IBL spherical harmonics from the top-mip
  faces (a port of Babylon's CubeMapToSphericalPolynomialTools.ConvertCubeMapToSphericalPolynomial)
  and returns the 9×3 polynomial coefficients to JS.

Apps/Playground/Scripts/config.json: re-enables the six tests this unblocks (NMEGLTF,
Anisotropic, Clear Coat, PBRMetallicRoughnessMaterial, PBRSpecularGlossinessMaterial, PBR).

Why this shape

These scenes load a prefiltered environment via a single .dds. WebGL gets the cube faces and
computes the spherical harmonics on the CPU through the texture-loader path. On native that path
is unavailable: _uploadDataToTextureDirectly / _uploadCompressedDataToTextureDirectly and cube
_readTexturePixels are unimplemented stubs. The .dds also stores no SH (Babylon computes it from
the faces). So the SH must be computed from the decoded faces, in native C++, alongside the upload —
no .env swap, no JS polyfill, no GPU readback.

Paired Babylon.js change

• [Native] Load single-file .dds/.ktx/.ktx2 cubemaps on the native engine Babylon.js#18560 wires the native createCubeTexture dispatch for single-URL containers
  and sets _sphericalPolynomial from the returned coefficients.

Draft: depends on a babylonjs dependency bump that includes #18560. CI will pass once that is in
place. Verified locally against a from-source babylon.max.js built with #18560 (see below).

Verification (local)

• The six re-enabled tests run via the normal path (no --include-excluded) and pass, matching their
  reference images. The SH integration is correct (total solid angle = 4π).
• Scope note: Prepass SSAO + depth of field also references a .dds env — its cube now loads, but it
  still fails on unrelated SSAO/DoF/prepass differences, so it remains excluded (separate issue).