Add TAIP-19: Fee Allocation specification

[iamonuwa]

Summary

Introduces TAIP-19 for fee allocation among TAP agents.
Defines FeeReceipt and FeeBatch message types for allocating, recording, and settling service fees.
Ensures predictable, auditable incentives for agents involved in authorization, routing, and execution while maintaining composability with existing TAP flows.

Key Features

• Fee Types: Defines Authorization, Routing, and Execution fees.
• Message Types: FeeReceipt (single obligation) and FeeBatch (aggregated obligations).
• Signing and Verification: Uses EIP-712 signatures bound to CAIP-10 identifiers.
• Governance Parameters: Provides configurable limits, supported units, and reporting requirements.
• Settlement-Agnostic: Supports off-chain fiat, custodial ledgers, or blockchain payments without prescribing rails.

Implementation Details

• FeeReceipt captures payer, payee, fee type, amount, unit, reference, and timestamp.
• FeeBatch aggregates multiple FeeReceipts with a Merkle root for auditability.
• Replay prevention enforced via nonces or unique references.
• Explicitly out of scope: settlement rails, conversion into settlement assets, and dispute resolution.

Test Plan

• Validate FeeReceipt generation and EIP-712 signature verification.
• Ensure replay protection by rejecting duplicate nonces.
• Aggregate multiple FeeReceipts into FeeBatch and verify Merkle root inclusion.
• Confirm FeeBatch totalAmount matches the sum of included receipts.