[M19-IMPL-04] Zcash shielded pool integration

Description

Integrate with Zcash shielded pools for full cross-chain privacy.

Why Zcash?

• Battle-tested: 8+ years of shielded transactions
• Full Privacy: Sender, amount, recipient all hidden
• Halo2: Proof system for composition research

Tasks

• Set up Zcash RPC client (zcashd/zebrad)
• Implement shielded deposit (t-addr → z-addr)
• Implement shielded withdrawal (z-addr → t-addr)
• Handle viewing key export/import
• Test on Zcash testnet

Integration Architecture

┌─────────────────┐     ┌─────────────────┐     ┌─────────────────┐
│   Solana/ETH    │────▶│  Zcash Shielded │────▶│   NEAR/Other    │
│   (SIP stealth) │     │     Pool        │     │   (SIP stealth) │
└─────────────────┘     └─────────────────┘     └─────────────────┘
        │                       │                       │
        ▼                       ▼                       ▼
   Stealth addr          Full shielded            Stealth addr
   + commitment         (t→z→z→t route)          + commitment

Zcash RPC Methods

// Deposit to shielded
await zcash.z_sendmany(tAddr, [{
  address: zAddr,
  amount: 1.0,
  memo: encryptedMetadata
}])

// Withdraw from shielded
await zcash.z_sendmany(zAddr, [{
  address: destTAddr,
  amount: 0.99
}])

Acceptance Criteria

• Shielded deposits work
• Shielded withdrawals work
• Viewing keys sync correctly
• Testnet E2E test passes

Parent

• Epic: #825

Start by reviewing the Zcash RPC methods and the integration architecture described in the issue, then identify the project entry points for the TypeScript RPC client. Work through shielded deposits, withdrawals, viewing-key synchronization, and Zcash testnet coverage. Done means deposits and withdrawals work, viewing keys sync correctly, and the testnet end-to-end test passes.