Seed content: Quant exponents & roots lesson + 7 questions

[DMDaudio]

What does this PR add or change?

Adds a new Quant → Arithmetic → exponents-roots bundle, a previously uncovered but high-frequency GMAT Focus Quant topic:

• 1 lesson (quant-arithmetic-exponents-roots) — power rules, negative/zero/fractional exponents, radical simplification, and factoring common powers, with worked examples and a common-traps section.
• 7 problem-solving questions (0017–0023) with an easy→hard progression:
  • 0017 (easy) — product/quotient power rules
  • 0018 (easy) — negative & zero exponents
  • 0019 (medium) — solving a same-base equation
  • 0020 (medium) — fractional exponents / roots
  • 0021 (medium) — simplifying and adding radicals
  • 0022 (hard) — factoring a sum of equal powers
  • 0023 (hard) — ratio of powers, (x−y)/y
• Each question's distractors are keyed to a specific common error (multiplying vs. adding exponents, merging unlike radicals, treating a sum of powers as a new base, dropping a −1 term).
• Choice-level math is wrapped in \( \) so the study site renders it via KaTeX.
• Rebuilt docs/data/content.json (now 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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