Assets: KTX2 container replaces .amtex (managed reader and writer) #31

Current State

Cooked textures are .amtex (TextureBlobWriter.cs, TextureBlob.cs). The Android plan makes KTX2 the texture container on every platform: BCn/RGBA8 payloads on desktop, ASTC on Android, one vkFormat per per-target file, no Basis transcoding at runtime. The MIT ktx2 Zig library (a reader) is the porting reference; existing .NET packages are native libktx bindings and are not used.

Tasks

• Port the reader to C#: header, level index, DFD, key/value data; supercompressionScheme = 0 only in cooked files (OAP handles compression)
• C# writer for the formats Turian bakes; Turian data goes in namespaced key/value entries, import settings stay in .meta
• Importer writes primary.<target>.ktx2; bump the importer version so caches rebuild; no permanent .amtex read path
• Runtime loads KTX2 through IContentSource
• Basis-bearing KTX2 as an import source is deferred

Acceptance Criteria

• Round-trip tests per format
• Cooked files pass ktx validate (KTX-Software) in CI
• Existing projects re-import transparently

Related

• Android plan task A11 · seniority Mid · mergeable now (.NET 10)
• Supersedes the earlier Basis-transcoder scope of this issue; DFD and fixtures: Assets: KTX2 data format descriptors and validated fixtures #247
• Platform roadmap: Platform roadmap research: Steam Deck, Android, iOS, Web #100