Split texture samples and add advanced texturing

[jonassorgenfrei]

Summary

• split the former combined Texture target into Texture_Basics, Texture_Compression, and Texture_VirtualTexturing
• keep introductory image loading and blending focused in the basics sample
• demonstrate DXT1/BC1 and ASTC 4x4 uploads with explicit capability diagnostics and RGBA8 fallbacks
• demonstrate bounded virtual-texture streaming with mip-aware residency and pinned coarse fallbacks
• provide focused controls and documentation for every sample

Validation

• configured with CMake successfully
• built all three targets in Release with zero warnings and errors
• runtime-smoke-tested all three executables and shader paths

Closes #7
Closes #42