Add combinatorics bundle to Quant counting-probability

[DMDaudio]

What does this PR add or change?

Adds first-time coverage of the empty quant / counting-probability / combinatorics subtopic:

• 1 lesson — quant-counting-probability-combinatorics: fundamental counting principle, permutations vs. combinations, the block method for adjacency restrictions, and complementary counting for "at least one."
• 6 original questions (0017–0022), easy → hard:
  • 0017 (easy) — fundamental counting principle (sandwich choices)
  • 0018 (easy) — basic permutation (arrange 5 distinct books)
  • 0019 (medium) — single-group combination (committee of 3 from 8)
  • 0020 (medium) — two-group committee (exactly 2 men + 2 women)
  • 0021 (hard) — arrangement with an adjacency restriction (block method)
  • 0022 (hard) — "at least one" via the complement

Every question has a worked explanation, two hints, and distractors keyed to specific common errors (add-vs-multiply, permutation-vs-combination, forgetting a block's internal order, reporting the complement, and the classic "at least one" over-count). Rebuilt docs/data/content.json via node scripts/build.mjs — 0 warnings.

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.

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