Add Quant number-properties: odds/evens/signs lesson + 5 questions

[DMDaudio]

What does this PR add or change?

Adds the quant / number-properties / odds-evens-signs subtopic, which previously had no lesson and no questions in the taxonomy:

• 1 lesson — "Odds, Evens & Signs": parity of sums/differences and products, consecutive-integer parity ((k(k+1)) is always even), sign rules for products/quotients (even/odd count of negatives), and the zero-is-even caveat.
• 5 problem-solving questions with worked explanations and hints, spanning a difficulty progression:
  • 0017 (easy) — parity of an expression given (x+y) odd; trap: (x-y) shares parity with (x+y).
  • 0018 (easy) — sign reasoning: (mn>0) and (m+n<0) ⇒ both negative.
  • 0019 (medium) — always-even via consecutive integers (k^2+k=k(k+1)).
  • 0020 (hard) — negative product of three integers; "exactly two negative" CANNOT hold.
  • 0021 (medium) — dropping odd coefficients: (3p+5q) even ⇒ (p,q) same parity.
• Rebuilt docs/data/content.json via scripts/build.mjs (now 37 questions, 9 lessons; Quant: 21 questions, 6 lessons). Build runs clean with no 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.

🤖 Generated with Claude Code

https://claude.ai/code/session_01SqS4XiZnt8Ar6y1FM4c1uW

---

Generated by Claude Code