Add interactive practice blocks

[sfw]

Summary

• add a typed interactive practice block contract with multiple-choice and deferred reasoning support
• normalize markdown-style practice output into structured blocks and stream full interactive blocks through SSE
• capture learner interaction observations and render the new learner-side interaction flow

Verification

• uv run ruff check src/ tests/
• uv run pytest
• cd frontend && npm test
• cd frontend && npm run lint
• cd frontend && npm run build