feat: expand quiz bank to 112 questions (DOJ-2435)

[dbejarano820]

Summary

• Expanded quiz bank from ~22 to 112 questions
• Coverage for all concepts in concept-tree.json
• Mix of multiple-choice and free-response formats
• Belt-appropriate difficulty distribution (40% white/yellow, 40% orange/green, 20% blue+)
• Teaching explanations for all questions

Test plan

• jq . data/quiz-bank.json > /dev/null validates
• 112 total questions
• All concept-tree keys have quiz coverage
• Questions have plausible wrong answers and explanations

Closes DOJ-2435

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[dbejarano820]

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[bitfalt]

Reviewed this in depth and found two functional issues: the selector was still choosing the first belt-compatible static question without respecting the requested quiz_format, and six new questions used an unsupported purple belt so they were effectively dead data. I also tightened the selector to treat lower-belt questions as eligible for higher belts, which makes the larger bank much more usable.

I patched this in my fork because the current account does not have write access to push directly to DojoCodingLabs/code-sensei:

• Fix branch: bitfalt:review/pr-11
• Fix commit: bitfalt@ba02b78

What I changed:

• Added a shared selector helper that filters first by compatible belt range and requested format, with a fallback to any compatible static question
• Replaced the unsupported purple belt values with brown

Validation: I verified locally that an orange-belt spaced/static run requesting free_response now returns a free-response static question, and that a black-belt run can reach the advanced brown-belt questions.