Seed Number Properties content: +6 questions, +3 lessons

[DMDaudio]

What does this PR add or change?

Fills a real gap in the bank: Quant number-properties had questions but no lessons. This adds a coherent study unit covering three subtopics, each with a lesson plus two problem-solving questions.

Lessons (3)

• quant-number-properties-factors-multiples — prime factorization, divisor counting, GCF/LCM rules, and the GCF × LCM = a × b identity.
• quant-number-properties-odds-evens-signs — parity of sums/products, the consecutive-integer trick (n(n+1) always even), and sign rules for products.
• quant-number-properties-remainders — the n = dq + r division equation, combining remainders, and simultaneous-condition problems.

Questions (6, all problem-solving, medium)

• ...factors-multiples-0017 — GCF/LCM identity to recover the missing integer.
• ...factors-multiples-0018 — LCM as "when do repeating cycles coincide" (lighthouse intervals).
• ...odds-evens-signs-0019 — "must be even" via n(n+1).
• ...odds-evens-signs-0020 — reading xy > 0 and x + y < 0 together (must-be-true).
• ...remainders-0021 — substitute n = 7k + 4 into 3n + 2.
• ...remainders-0022 — smallest value satisfying two remainder conditions.

Rebuilt the content index (docs/data/content.json): 32 → 38 questions, 8 → 11 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.

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https://claude.ai/code/session_015yeU7ESNtvyMQ8VhZ7U5sg

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