Expose KTX2 encoder thread count in CLI

[Xananthium]

Summary

• expose the existing GltfConverterOptions.Ktx2Threads setting as --ktx2-threads
• keep 0 as the unchanged one-thread-per-texture default
• reject negative values and positive values without --texture-format Ktx2
• document per-texture threading and the risk of oversubscription because tile conversion is already parallel

This is a KTX2-specific, backward-compatible seam related to #8; it does not attempt to cap the other parallel pipeline stages.

Validation

• dotnet test Obj2Tiles.sln --configuration Release --verbosity minimal
  • Obj2Tiles.Library.Test: 135 passed
  • Obj2Tiles.Test: 85 passed
• focused option contract: 5 passed
• real native libktx smoke conversion with --ktx2-threads 2: exit 0, tileset.json plus two B3DM outputs

[Copilot]

Pull request overview

This PR exposes the existing per-texture libktx encoder thread setting (GltfConverterOptions.Ktx2Threads) through the Obj2Tiles CLI as --ktx2-threads, with validation and documentation to avoid misuse and oversubscription while the tiling pipeline already runs in parallel.

Changes:

• Adds --ktx2-threads to CLI options and wires it into GltfConverterOptions.Ktx2Threads when --texture-format Ktx2 is selected.
• Validates that --ktx2-threads is non-negative and rejects positive values unless --texture-format Ktx2 is set.
• Documents the option and adds focused option-contract tests.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
README.md	Documents --ktx2-threads and explains oversubscription risk alongside existing KTX2 docs.
Obj2Tiles/Program.cs	Propagates Ktx2Threads into glTF conversion options and enforces CLI validation rules.
Obj2Tiles/Options.cs	Adds the --ktx2-threads option with default 0 and CLI help text.
Obj2Tiles.Test/Ktx2ThreadsOptionsTests.cs	Adds parsing/validation tests to lock down the CLI contract.
Obj2Gltf/GltfConverterOptions.cs	Updates the XML doc comment to match actual per-texture thread semantics.