Ktx2Texture class final

A KTX2 file, read down to what a texture upload needs: dimensions, the raw vkFormat its mip bytes are in, and each level's bytes.

vkFormat, not TextureFormat. ap-01 in doc/asset-pipeline-plan.md: this class used to carry the engine's own TextureFormat, and carrying it here would put flutter3d_hardware — and the Flutter SDK it needs — on this package's own dependency graph, the thing flatDartPackages exists to keep off it. Reading a container and knowing which pixel formats a specific GPU backend accepts are two different questions; this answers only the first, honestly, in the container's own vocabulary — the Khronos VkFormat numbers, not an engine's. flutter3d's own Ktx2Texture wraps this one and does the second: see its own doc comment for why the exact same wrapping act, with the exact same reasoning, is not new to this session.

See ktx2_format.dart for the layout and for exactly which files this stage refuses rather than misreads.

levels are views over the bytes passed to parse — ByteData.view has no alignment requirement, unlike Uint32List.view/Float32List.view, so unlike .f3d's loader this needs no copying fallback for an odd offset.