docs: define anime-ready art direction

[JOY (JOY)]

Summary

• Define the alpha art style as Anime-Ready Semi-Real with a clean / controlled PBR material pipeline.
• Add reference examples, negative examples, asset search terms, asset rejection rules, and scalable PC/Web/mobile quality tiers.
• Sync the GDD, alpha GDD, vertical slice spec, decision register, roadmap, and changelog.

Local Review

• Local code-review fallback: APPROVED. Docs-only change; no Unity/Nakama authority boundary changes; no AGENTS.md or .claude/CLAUDE.md project-context drift.

Verification

• npx.cmd markdownlint-cli2 "docs/**/*.md" "*.md"
• Markdown local link resolver for changed docs
• rg -n "—|–|…" ... on changed files
• git diff --check