docs(privacy): add Privacy as a Core Principle wiki page

[asmortongpt]

Summary

Adds a new comprehensive wiki page at site/privacy/Privacy_as_a_Core_Principle.md for issue #1562.

Coverage:

• Why financial privacy matters: employer/client surveillance, merchant data, authoritarian surveillance, criminal targeting, behavioral profiling — establishes that privacy is a prerequisite for freedom, not cover for wrongdoing
• How Zcash implements privacy: ZK-proofs (zk-SNARKs, Halo 2), what a zero-knowledge proof proves vs. what it hides
• The three shielded pools: Sprout (deprecated), Sapling, Orchard — with introducing upgrade and proving system for each
• Shielded vs. transparent transactions: explicit table showing z→z, t→z, z→t, t→t with privacy levels and implications
• Unified Addresses: why they improve privacy automatically as sender wallets upgrade
• Viewing keys for selective disclosure: IVK, FVK, payment disclosure — what each reveals and appropriate use cases
• Privacy best practices: 6 specific practices (shielded addresses, shielding after exchange, separate wallets, memo caution, address rotation, verifying transaction type)
• Common mistakes table: with specific fix for each
• Privacy comparison: Zcash vs. CoinJoin, Monero, Tornado Cash — what makes ZK-proof-based privacy different
• Summary: actionable 4-point checklist

All sections have explicit {#id} anchors for direct linking as requested in the issue.

Closes #1562

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ZEC address (Unified Address):
u1l6vp9rqzaujmx57c8grgxstqcq4gchpw3rqxhgydttnj4nfqzuf7v8guzl2xhwn96qtnw46mp2fjvme6vaqkevhkf39djvx2q0uqx3r6smmvlzq5dzjc3wqdjx9f3c0r79qpmzxcfhjvujwwlnecf6xjxqgdts3