Add Number Properties lesson + 6 practice questions

[DMDaudio]

What does this PR add or change?

Seeds the previously lesson-less Number Properties topic. Adds one foundational lesson and six practice questions spanning its three subtopics with an easy→hard difficulty progression, then rebuilds the content index (docs/data/content.json: now 38 questions, 9 lessons).

• Lesson — quant-number-properties-factors-multiples: factors vs. multiples, prime factorization, divisor counting, GCD/LCM from primes, and the gcd × lcm = product identity.
• factors-multiples — counting multiples in a range (easy), GCD/LCM product identity (medium), perfect-square divisor count (hard).
• odds-evens-signs — parity "must be even" (easy).
• remainders — linear remainder 3n+5 mod 7 (medium), power cyclicity 7^100 mod 5 (hard).

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.

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