[codex] add zcash x402 binding

[Svaag]

Summary

Adds a custom x402 v2 Zcash binding for Hyrule Cloud.

• Implements exact native ZEC requirements using CAIP-2 Zcash network IDs and slip44:133.
• Adds merchant-hosted Zcash invoice persistence and verification through wallet-visible z_viewtransaction outputs.
• Wires Zcash invoice minting into the existing PaymentGate, including PAYMENT-REQUIRED, PAYMENT-SIGNATURE, and PAYMENT-RESPONSE handling.
• Adds /x402/zcash/supported, /verify, and /settle facilitator endpoints.
• Advertises Zcash in /v1/payments/networks and /.well-known/x402.json when enabled.
• Commits the original implementation plan at docs/plans/2026-07-01-zcash-x402-v2-binding.md.

Impact

Operators can enable client-broadcast Zcash payments with a merchant-hosted verifier backed by a zcashd wallet view. Shielded ZEC is treated differently from public-chain rails: txid-only public verification is not assumed.

Validation

• uv run ruff check hyrule_cloud tests/test_zcash_x402.py tests/test_payment_gate_x402.py tests/test_payments_networks.py
• uv run pytest

[Svaag]

Original implementation plan committed in this PR:

docs/plans/2026-07-01-zcash-x402-v2-binding.md

That file captures the Zcash-specific x402 architecture, including the shielded-payment verification caveat, client-broadcast flow, facilitator behavior, invoice binding, replay protection, and phased MVP rollout.