docs: adopt MmdMaterialAPI policy

[snkmcb]

Summary

• adopt MmdMaterialAPI as the formal MMD material schema while preserving existing mmd:material:* properties
• define mmdSchema and mmdImaging boundaries for hydra-toon
• update Phase 8 roadmap, package contracts, capability matrix, and source mapping
• keep PreviewSurface and MaterialX as generic fallbacks and defer a shared ToonMaterialAPI

Validation

• documentation consistency checker: 41 Markdown files passed
• git diff --check
• verified no hydra-mtoon references remain