Add combinatorics bundle: lesson + 7 questions (quant/counting-probability)

[DMDaudio]

What does this PR add or change?

Fills the previously empty counting-probability/combinatorics subtopic (it had zero questions and no lesson, while probability already had coverage).

Adds:

• 1 lesson — quant-counting-probability-combinatorics ("Combinatorics: Counting Without Listing"): fundamental counting principle, factorial, permutations (nPr), combinations (nCr), arrangements with identical items, and circular arrangements, with worked examples, common traps, and takeaways.
• 7 original questions (quant-counting-probability-combinatorics-0017 → 0023) in an easy→hard progression:
  • 0017 (easy) — fundamental counting principle (multiply independent stages)
  • 0018 (easy) — arranging n distinct items (n!)
  • 0019 (medium) — permutations, distinct roles (nPr)
  • 0020 (medium) — combinations, unordered committee (nCr)
  • 0021 (hard) — restricted committee, exactly 2 men + 2 women
  • 0022 (hard) — arrangements of BANANA (identical letters)
  • 0023 (hard) — circular seating, (n−1)!

Each question has a worked explanation written for someone who got it wrong, two hints, and distractors keyed to specific common errors (adding vs. multiplying, permutation vs. combination, dividing by the wrong factorial, ignoring/mis-handling identical items, treating a circle as a row).

Regenerated docs/data/content.json via node scripts/build.mjs — zero warnings (39 questions, 9 lessons).

Type of change

• New question(s)
• New lesson(s)
• Fix to existing content (answer / explanation / tag)
• Tooling / docs / schema

Author checklist

• This is my own original work — not copied or paraphrased from any real GMAT question or prep material.
• Front-matter follows CONTENT_SCHEMA.md (id matches the filename; topic/subtopic exist in curriculum.md).
• I solved every question myself and confirmed exactly one answer choice is correct.
• Each explanation would help someone who got the question wrong, and names the trap where relevant.
• status is in-review. I did not set my own content to verified.

For reviewers

• I solved the question(s) without reading the keyed answer first and got the same answer.
• Exactly one choice is correct; distractors are plausible.
• Explanation and tags (section / topic / difficulty) are accurate.
• If approving: bumped status to verified and added my handle to reviewers.

🤖 Generated with Claude Code

https://claude.ai/code/session_01FJ3gK9RbjNA17dZu2h2HCY

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