Add Number Properties content: remainders lesson + 4 questions

[DMDaudio]

What does this PR add or change?

Expands the thin Quant → Number Properties area, which previously had no lesson and only one covered subtopic (factors-multiples).

• New lesson — Remainders & Modular Thinking (quant-number-properties-remainders): the n = qd + r identity, remainder arithmetic under addition/multiplication, combining coprime conditions, and units-digit cyclicity.
• 4 new original problem-solving questions (quant ids 0017–0020, continuing the per-section counter):
  • remainders-0017 — substitute the qd + r form (medium)
  • remainders-0018 — simultaneous remainder conditions mod 15 (hard)
  • odds-evens-signs-0019 — parity / "must be even" reasoning; opens the new odds-evens-signs subtopic (medium)
  • remainders-0020 — units digit of (7^{100}) via cyclicity (hard)
• Rebuilt docs/data/content.json via scripts/build.mjs (36 questions, 9 lessons; no build 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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