Add exponents & roots bundle (quant/arithmetic)

[DMDaudio]

What does this PR add or change?

Adds a full bundle for the previously-empty quant / arithmetic / exponents-roots subtopic:

• 1 lesson — quant-arithmetic-exponents-roots: laws of exponents (product, quotient, power-of-a-power), zero/negative/fractional exponents, simplifying radicals, solving common-base equations, and factoring out a common power.
• 7 original questions (0017–0023), easy → hard:
  • 0017 (easy) — multiplying same-base powers (add exponents)
  • 0018 (easy) — power of a power (multiply exponents)
  • 0019 (medium) — negative exponent on a fraction (reciprocal)
  • 0020 (medium) — fractional exponent as root-and-power ((27^{2/3}))
  • 0021 (medium) — simplifying a radical ((\sqrt{50}))
  • 0022 (hard) — solving (4^{x+1}=8^{x-1}) by common base
  • 0023 (hard) — simplifying (\frac{3^{12}-3^{10}}{3^{10}}) by factoring

Each question has a worked explanation, two hints, and distractors keyed to specific common errors (add-vs-multiply exponents, negative-exponent sign confusion, wrong root/power order, pulling the perfect square out instead of its root, 8 = 2^4 slip, forgetting to cancel the denominator). Correct answers are spread across A–E. docs/data/content.json regenerated via node scripts/build.mjs with zero 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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