Add Quant mixtures lesson + 5 practice questions

[DMDaudio]

What does this PR add or change?

Fills the empty word-problems / mixtures subtopic in Quant. Adds one concept lesson and five difficulty-progressing Problem Solving questions, then rebuilds the content index.

• Lesson quant-word-problems-mixtures — "Mixtures & Solutions": amount-conservation (concentration × volume), water as 0% / pure component as 100%, dilution, replacement problems, and alligation for two-way blends, with worked examples, common traps, and key takeaways.
• Questions quant-word-problems-mixtures-0017–0021, progressing easy → hard:
  • 0017 (easy) — combining two solutions; trap = averaging the percentages.
  • 0018 (medium) — adding pure acid; trap = forgetting the volume grows too.
  • 0019 (medium) — dilution with water; trap = losing the fixed component amount.
  • 0020 (hard) — drain-and-replace; trap = treating drained volume as pure component.
  • 0021 (hard) — alligation ratio; trap = reversing the mixing ratio.
• Index docs/data/content.json rebuilt via node scripts/build.mjs (32 → 37 questions, 8 → 9 lessons; build reports no warnings).

Type of change

• New question(s)
• New lesson(s)
• Fix to existing content (answer / explanation / tag)
• Tooling / docs / schema (regenerated content 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).
• 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.

🤖 Generated with Claude Code

https://claude.ai/code/session_014KvdUA1eEHtttBQcWVCvLP

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