Add opt-in Zstd supercompression for UASTC KTX2

[Xananthium]

Summary

• add opt-in --ktx2-zstd-level 1-22 for UASTC KTX2 output
• keep 0 as the default, preserving the existing output path
• call the existing libktx ktxTexture2_DeflateZstd API after ktxTexture2_CompressBasisEx
• reject out-of-range levels, non-KTX output, and ETC1S/BasisLZ combinations
• document the UASTC-only constraint and memory caution for levels above 20

The range and ETC1S restriction follow the official KTX tools contract. This PR is independent of #102 and does not include its thread-count CLI change.

Validation

• dotnet test Obj2Tiles.sln --configuration Release --verbosity minimal
  • Obj2Tiles.Library.Test: 135 passed
  • Obj2Tiles.Test: 87 passed
• focused option contract: 7 passed
• real native conversion using the vendored Windows x64 libktx v4.4.2: exit 0, tileset.json and two B3DM outputs
• vendored ktx.dll is byte-identical to the official v4.4.2 release DLL
• all 12 embedded KTX2 images report supercompressionScheme = 2 (Zstd)
• official Khronos ktx v4.4.2, for every embedded image:
  • validate --warnings-as-errors --gltf-basisu: passed
  • transcode --target rgba8: passed
  • validation of transcoded KTX2: passed

[Copilot]

Pull request overview

Adds an opt-in CLI option to apply Zstandard supercompression to UASTC KTX2 texture output, wiring the setting from the CLI through GltfConverterOptions into the libktx encoding path, and documenting the constraint/usage in the README.

Changes:

• Add --ktx2-zstd-level 0|1-22 option (default 0 disabled) and validate it in CLI option checks.
• Plumb the option into GltfConverterOptions and invoke ktxTexture2_DeflateZstd after ktxTexture2_CompressBasisEx when enabled.
• Add focused option-contract tests and documentation updates for the UASTC-only Zstd constraint.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
README.md	Documents the new --ktx2-zstd-level option, constraints, and an example invocation.
Obj2Tiles/Program.cs	Validates --ktx2-zstd-level constraints and passes the value into GltfConverterOptions for tiling.
Obj2Tiles/Options.cs	Adds the new CLI option definition and help text.
Obj2Tiles.Test/Ktx2ZstdOptionsTests.cs	Adds tests covering parsing and validation rules for the new option.
Obj2Gltf/Ktx2/Ktx2Encoder.cs	Adds P/Invoke for ktxTexture2_DeflateZstd and applies it conditionally for UASTC KTX2 output.
Obj2Gltf/GltfConverterOptions.cs	Adds Ktx2ZstdLevel to carry the setting from CLI into the encoder.