Docs: add Magic Exam Hall game design roadmap

[SilverSupplier]

Summary

• Adds docs/GAME_DESIGN.md for the Magic Exam Hall game concept, world-drawing spell input, spell grammar, floor structure, feedback, and report flow.
• Adds docs/PROJECT_ROADMAP.md to track implementation state and next roadmap items against the game direction.
• Updates README.md to explain the Magic Recognizer / Magic Exam Hall relationship and current docs reading order.

Validation

• node scripts/validate-doc-state.mjs -> validated 0 task documents against work queue
• npm test not run; docs-only change.
• npm run build not run; docs-only change.

Notes

• This PR intentionally contains docs only: README.md, docs/GAME_DESIGN.md, and docs/PROJECT_ROADMAP.md.
• Unity prototype/code changes are not included in this PR.

Closes #22