Add Exponents & Roots bundle: lesson + 7 questions

[DMDaudio]

What does this PR add or change?

Adds a coherent bundle for the previously-uncovered quant / arithmetic / exponents-roots subtopic (zero questions and no lesson existed before):

• 1 lesson — quant-arithmetic-exponents-roots ("Exponents & Roots"): the exponent laws (add vs. multiply), zero/negative exponents, fractional exponents as roots, simplifying and adding radicals, and solving exponential equations by matching bases.
• 7 original problem-solving questions (0017–0023, the next unused Quant sequence), with an easy→hard progression:
  • 0017 (easy) — product of powers, \(3^4 \cdot 3^2\)
  • 0018 (easy) — power of a power, \((2^3)^2\)
  • 0019 (easy) — simplify \(\sqrt{72}\)
  • 0020 (medium) — zero & negative exponents, \(4^0 + 2^{-2}\)
  • 0021 (medium) — fractional exponent, \(27^{2/3}\)
  • 0022 (hard) — solve \(2^{x+3} = 8^{x-1}\) by common base
  • 0023 (medium) — add radicals, \(\sqrt{50} + \sqrt{18}\)

Every question has a worked explanation, two hints, and distractors keyed to specific common errors (multiply-vs-add exponents, reading a negative exponent as a negative number, root-then-power order, adding under the radical, and forgetting to distribute a converted exponent). Correct-answer letters are spread across A–E.

node scripts/build.mjs regenerated docs/data/content.json with 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.

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