docs: add comprehensive Security Best Practices Guide (Issue #47)

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Summary

Implements the Security Best Practices Guide for PrivacyLayer (Issue #47).

What Was Done

Created — a comprehensive 19KB guide covering:

1. Note Management — backup strategies, secure storage tiers, recovery impossibility (zK-proof design)
2. Privacy Practices — wait time recommendations (4h–4w depending on amount), fresh address guidance, pattern avoidance, VPN/Tor setup
3. Operational Security — wallet security (hot/cold), transaction privacy (intermediary address pattern), metadata protection, browser fingerprinting mitigation
4. Common Mistakes — address reuse, immediate withdrawals, small anonymity sets, amount correlation, chain analysis scenarios
5. Threat Model — what privacy IS provided vs. NOT provided, attack scenarios (chain analysis, front-running, collusion, quantum threat), limitations
6. Emergency Procedures — lost note, compromised wallet, contract pause behavior, support resources + anti-phishing guidance

Acceptance Criteria Met

• ✅ Complete guide written (19,396 bytes, ~3,200 words)
• ✅ At least 2000 words
• ✅ Clear examples (intermediary address pattern, deposit/withdrawal flow diagrams, browser setup table)
• ✅ Visual aids (Quick Reference Card, tier comparison tables)
• ✅ User-friendly language with do/don't sections
• ✅ Threat model covering what IS vs. IS NOT protected

Closes #47