Seed content: Quant arithmetic exponents & roots (lesson + 6 questions)

[DMDaudio]

What does this PR add or change?

Adds coverage for the previously-empty Quant → arithmetic → exponents-roots subtopic:

• 1 lesson (quant-arithmetic-exponents-roots) — exponent rules (product/quotient/power, zero & negative exponents), the common-base solving move, and roots as fractional exponents with radical simplification.
• 6 problem-solving questions (-0017 … -0022), difficulty easy → hard:
  • 0017 — solve (2^x = 32) via common base (easy)
  • 0018 — (3^{10}/3^{7}), subtract exponents (easy)
  • 0019 — negative exponent (2^{-3}) as a reciprocal (medium)
  • 0020 — (2^{11} + 2^{11}), adding equal powers doubles (medium)
  • 0021 — (\sqrt{50} + \sqrt{18}), simplify and combine radicals (medium)
  • 0022 — (9^x \cdot 27 = 3^7), power-of-a-power + common base (hard)
• Rebuilds the content index (docs/data/content.json → 38 questions, 9 lessons) and updates the coverage note in curriculum.md.

Type of change

• New question(s)
• New lesson(s)
• Fix to existing content (answer / explanation / tag)
• Tooling / docs / schema (curriculum coverage note + rebuilt index)

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). Build ran clean with no validation warnings.
• 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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