feat: add Canton Network payment method specification

[anilkaracay]

What

Adds the "canton" payment method specification for MPP, enabling payments in USDCx (Circle's USDC-backed stablecoin) on Canton Network using the CIP-56 TransferFactory mechanism.

Why

Canton Network is the institutional blockchain operated by Digital Asset, with participants including DTCC, Goldman Sachs, JPMorgan, and BNP Paribas. There is no existing MPP method for Canton. This specification enables the growing Canton DeFi ecosystem to integrate with the Machine Payments Protocol.

Key differentiators from EVM-based methods:

• Client-submitted transactions — Canton clients execute transfers directly on the ledger, then present the update ID as credential (vs. signing a tx for the server to broadcast)
• Deterministic finality — No confirmation wait, no reorgs
• Sub-transaction privacy — Payment details visible only to sender and receiver
• String-based amounts — Up to 10 decimal places, no floating point

Reference Implementation

• npm: @caypo/mpp-canton v0.2.0
• Source: github.com/anilkaracay/Caypo
• Live gateway: mpp.caypo.xyz — 17 services, 46 endpoints
• Tests: 312 passing, 14 E2E on Canton DevNet (Splice v0.5.12)
• Docs: caypo.xyz/docs

Spec Contents

• IETF format following examples/method-template.md
• Request schema with Canton-specific fields (party ID, network, Numeric 10 amounts)
• Credential schema with updateId/completionOffset/sender/commandId
• 8-step client settlement procedure (CIP-56 TransferFactory.Transfer)
• 8-step server verification procedure
• Error code mapping to MPP problem types
• Security considerations: privacy, replay protection, TransferPreapproval, amount precision
• IANA payment method registration
• Working examples with HTTP headers

Disclosure

AI assistance was used in drafting this specification. All technical details have been verified against the Canton JSON Ledger API v2 documentation and tested with a live Canton DevNet deployment.

[brendanjryan]

@anilkaracay -- thanks for submitting! Are you officially representing canton or have endorsement on this PR from the maintainers of canton? we would like to ensure that payment method specs can be owned by the ultimate operators of the payment method