feat: diversify spaced repetition reviews

[uniStark]

Summary

• Add deterministic concept-aware review selection with stable priority ordering and round-robin coverage.
• Reserve bounded same-concept transfer slots only when the due pool exceeds the daily limit, while preserving deferred due items.
• Add optional concept_id metadata, legacy compatibility mappings, and selector output with grading context.
• Validate review quality, unknown review IDs, duplicate payload entries, and preserve concept metadata in the updater.
• Align SM-2 rounding/mastery transitions and synchronize review documentation and schemas.

Verification

• python3 -m unittest discover -s tests -p 'test_*.py' — 36 tests passed.
• Python 3.8 AST parsing and bytecode compilation passed.
• JSON validation and git diff --check passed.
• Live learner databases were not modified; selector was verified against a read-only snapshot.

🤖 Generated with Claude Code