ISSUE-014 — Cross-topic interleaving scheduler

[shivaram19]

• Functional surgeon view: Daily quizzes mix topics rather than drilling one topic, to improve retention.
• Acceptance criteria:
  1. No single topic occupies >60% of a 5-question set when ≥3 topics have attempts.
  2. Interleaving ratio is configurable.
• Dependencies: ISSUE-012 (math PWA) or ISSUE-001 (nursing selector).