docs: redesign Playground around ontology-first compilation

[windforce19820520-ai]

Summary

• add a repository-grounded, ontology-first Playground architecture on the RC.4 audit baseline
• make declared reference-role semantics ScenarioPack-derived and require reference isolation to survive Prompt Guard into the accepted Provider request
• add ready-made pose references to Virtual Try-On and Cosplay without adding a pose extractor, skeleton generator, or editor
• replace the “free Provider” constraint with approved Provider/profile selection, including Seedream and Grok Imagine candidates
• define request-scoped BYOK handling and split future implementation into PR 0/A/B/C/D
• add both documents to the documentation index

Boundaries

• documentation only
• no Core, contract, fixture, Provider, or Playground code changes
• no deployment, credentials, release, or tag changes
• real Provider calls: 0

Verification

• repository validation passed (46 required files)
• changed-document local-link audit passed (0 missing targets)
• git diff --check passed