feat: add Zcash (shielded) network support

typescript/packages/mechanisms/zcash/README.md

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+# Zcash (Shielded) Network Support for x402
+
+This adds Zcash as a supported network in the x402 protocol, enabling shielded agent-to-agent payments.
+
+## Why Zcash
+
+x402 currently supports 8 chains (Base, Solana, Aptos, Stellar, Avalanche, Polygon, Sei, SKALE). All use transparent ledgers. Zcash adds the first shielded settlement option: agents can pay for HTTP resources without revealing transaction amounts or addresses on-chain.
+
+This is relevant for:
+- Agent treasury flows where payment history should not be public
+- x402 payments that require privacy compliance (GDPR, financial regulation)
+- Selective disclosure: prove an agent paid without revealing how much or to whom
+
+## Architecture
+
+Zcash is not an EVM chain. It requires a new mechanism module (parallel to `evm`, `svm`, `aptos`, `stellar`):
+
+```
+typescript/packages/mechanisms/zcash/
+  src/
+    index.ts          # mechanism entry point
+    verify.ts         # verify payment via Zebra RPC
+    settlement.ts     # memo-based payment matching
+```
+
+Payment verification uses the shielded memo field: the x402 payment ID is encoded in the Zcash transaction memo, and the facilitator verifies the payment by checking the memo against the expected payment ID via Zebra RPC.
+
+## Reference implementation
+
+The full Zcash x402 facilitator is available as a standalone npm package:
+- [@frontiercompute/zcash-402](https://www.npmjs.com/package/@frontiercompute/zcash-402) (MCP server, 4 tools)
+
+This PR adds the network definition and mechanism stub. The full mechanism delegates to zcash-402 for payment construction and verification.
+
+## Network configuration
+
+- **CAIP-2:** `zcash:mainnet` (Zcash mainnet, chain ID derived from genesis block)
+- **RPC:** Zebra JSON-RPC (default `http://127.0.0.1:8232`)
+- **Asset:** ZEC (8 decimals, native)
+- **Memo encoding:** x402 payment ID in Zcash encrypted memo field
+- **Attestation:** Optional ZAP1 Merkle receipt for payment auditability

typescript/packages/mechanisms/zcash/src/index.ts

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+/**
+ * Zcash (shielded) mechanism for x402.
+ *
+ * This is a stub that defines the Zcash network configuration
+ * for the x402 protocol. The full payment construction and
+ * verification logic is in @frontiercompute/zcash-402.
+ *
+ * Zcash uses encrypted memos for payment ID matching,
+ * unlike EVM/SVM which use on-chain event logs.
+ */
+
+export const ZCASH_NETWORK = {
+  name: "zcash",
+  caip2: "zcash:mainnet",
+  rpcDefault: "http://127.0.0.1:8232",
+  asset: {
+    symbol: "ZEC",
+    decimals: 8,
+    native: true,
+  },
+  settlement: "shielded-memo",
+  facilitatorPackage: "@frontiercompute/zcash-402",
+} as const;
+
+export type ZcashPaymentProof = {
+  txid: string;
+  memo: string;
+  amount_zat: number;
+  attestation_leaf?: string;
+};
+
+export function isZcashNetwork(caip2: string): boolean {
+  return caip2.startsWith("zcash:");
+}