feat(withdraw): ZK-073 redesign relayer binding and zero-relayer semantics

[s6pa1rta3n-lab]

Summary

Wave Issue Key: ZK-073
Resolves #349

Redesigns relayer binding and zero-relayer semantics across Noir circuits, SDK, and Soroban smart contracts:

• Canonical relayer encoding: Absent relayer is represented as None in SDK and encoded as canonical 0x00 32-byte field element with fee=0.
• Circuit enforcement: Bidirectional validation in circuits/lib/src/validation/relayer.nr enforces that zero relayer requires zero fee, and non-zero fee requires non-zero relayer.
• Contract enforcement: validate_relayer_and_fee in Soroban contract rejects malformed relayers and inconsistent fee structures.
• Regression test suite: Distinguishes absent relayer from malformed relayer across SDK, Noir circuits, and Soroban contracts.

Verification

• Noir circuits: nargo check --workspace & nargo test --package withdraw (35/35 tests pass).
• TypeScript SDK: npm run build & npm test (57/57 suites pass, 860/860 tests pass).
• Soroban Contracts: cargo test (93/93 tests pass).
• Ticket Verification: node scripts/zk_ticket_check.mjs --issue-key ZK-073 --run (all derived ticket checks passed).

Payout Routing

• EVM (Base/Arbitrum/Polygon/ETH): 0xF46C9F6d70C50BF81ef3588AB523a90a594a2F89
• Stellar: GCL6OXAMLD75BMTINA6EMRUDWK5THQUSHMYNLSNBCJAPZJHNYJTUNIBC