Support smart-wallet Solana payments in exact SVM (SWIG first)

[romeo4934]

Summary

• enable smart-wallet Solana payments in exact SVM, starting with SWIG wallets
• keep direct top-level SPL TransferChecked support as the default exact SVM payment path
• add built-in support for SWIG-wrapped exact payments in the Go, TypeScript, and Python SVM facilitators
• verify SWIG SignV2 and SubAccountSignV1 by decoding them into the same canonical transfer details used for direct payments
• update the SVM README and exact SVM spec to document smart-wallet compatibility through known wrapped payment programs

Why

Today the stock exact SVM facilitator only accepts transactions whose third top-level instruction is a direct SPL Token or Token-2022 TransferChecked. That blocks smart-wallet Solana payment flows where the exact same SPL payment is wrapped inside a higher-level wallet instruction.

This shows up in practice with wallets such as SWIG: the payment is still an exact SPL token transfer, but it is expressed through a smart-wallet instruction rather than a direct top-level TransferChecked.

This PR keeps the existing exact-scheme safety checks on:

• token program
• source token account
• mint
• destination ATA
• authority
• exact amount
• fee-payer safety

The change is that facilitators now recognize a small set of known wrapped payment programs in addition to the direct transfer shape. SWIG is the first built-in smart-wallet integration. Future wrappers such as Squads can be added explicitly using the same pattern.

Implementation

Go

• add built-in canonical transfer decoding for direct SPL transfers and SWIG-wrapped smart-wallet transfers
• update both V1 and V2 exact facilitators to use the same built-in transfer decoding path
• add real SWIG serialized transaction fixtures for SignV2 and SubAccountSignV1

TypeScript

• add the same built-in transfer decoding logic to the SVM exact facilitators
• update V1 and V2 facilitators to verify canonical transfer details from either direct or SWIG-wrapped payments
• add fixture-driven unit coverage for both SWIG transaction shapes

Python

• add the same built-in transfer decoding logic to the SVM exact facilitators
• update V1 and V2 facilitators to verify canonical transfer details from either direct or SWIG-wrapped payments
• add fixture-driven unit coverage for both SWIG transaction shapes

Tests

Added unit coverage in all three language implementations for:

• direct exact SVM payment verification still working as before
• SWIG SignV2 wrapped payments
• SWIG SubAccountSignV1 wrapped payments

Validation run locally:

• cd go && go test ./mechanisms/svm/... && go test ./...
• cd typescript/packages/mechanisms/svm && npm exec --yes pnpm@10.12.1 -- format && npm exec --yes pnpm@10.12.1 -- lint && npm exec --yes pnpm@10.12.1 -- test
• cd python && /opt/homebrew/bin/python3.11 -m black x402/mechanisms/svm/constants.py x402/mechanisms/svm/types.py x402/mechanisms/svm/utils.py x402/mechanisms/svm/exact/facilitator.py x402/mechanisms/svm/exact/v1/facilitator.py x402/tests/unit/mechanisms/svm/test_facilitator.py && /opt/homebrew/bin/python3.11 -m ruff check x402/mechanisms/svm/constants.py x402/mechanisms/svm/types.py x402/mechanisms/svm/utils.py x402/mechanisms/svm/exact/facilitator.py x402/mechanisms/svm/exact/v1/facilitator.py x402/tests/unit/mechanisms/svm/test_facilitator.py && /opt/homebrew/bin/python3.11 -m pytest x402/tests/unit/mechanisms/svm/test_facilitator.py

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge)
• I added a changelog fragment for user-facing changes

[romeo4934]

A bit of reviewer context: this PR intentionally takes a “known wrapped payment programs” approach rather than a generic extension hook.

The goal is to keep the exact SVM facilitator simple and predictable while unlocking smart-wallet payment flows that still resolve to the same canonical SPL transfer. SWIG is the first built-in case because it is already used in production-like flows and maps cleanly to the exact-scheme safety checks.

This means exact SVM now supports:

• direct top-level TransferChecked
• vetted wrapped payment programs that deterministically decode to the same canonical transfer intent

If other smart-wallet wrappers such as Squads need support later, they can be added explicitly following the same pattern.