feat: implement MPP seller charge workflows

[nkavian]

Summary

• Implement asynchronous MPP Seller setup and single-offer InFlow/Tempo charge handling through pympp's public intent and standalone payment interfaces.
• Select configured rails, bind platform validation responses to submitted credentials, and preserve broadcast idempotency and receipt fields.
• Document Seller account setup, transport ownership, receipt handling, and separate upstream limitations for amount conversion and subscription configuration.

Verification

Compared the Seller lifecycle with the Node implementation/tests and the platform controller. Exercised actual FastAPI challenge/retry handling, route binding, failure responses, cancellation, concurrent broadcasts, and stable retry keys. Run the shared Seller charge vectors; subscription cases are outside the approved scope.

make sync and make verify passed on Python 3.11, 3.12, 3.13, and 3.14, including strict typing, tests, distribution builds, and isolated package consumers.

Scope

No Seller subscriptions, composed InFlow offers, replacement route framework, or live payments. Upstream limitations are tracked separately in tempoxyz/pympp#268 and tempoxyz/pympp#269. Buyer subscription support is unchanged.