feat(tap): add optional Python seller verification

[nkavian]

Summary

• Add optional inflowpay[tap] Seller verification with Ed25519, trusted-key retrieval, caching, replay protection, and an asynchronous protected-handler entry point.
• Document request binding, resolver ownership, cancellation, and the separation between Agent recognition, account authorization, and payment.
• Add a runnable FastAPI example, real signed-request and loopback HTTP tests, an isolated TAP-only package consumer, and the shared TAP conformance adapter.
• Keep published-version collision checks on manual release verification and publication, not feature PR builds. Test the workflow's actual tag-validation shell against PR and release scenarios.

Verification

• make sync verify on Python 3.11, 3.12, 3.13, and 3.14, including source-distribution builds and isolated consumers.
• node --test scripts/conformance.test.mjs.
• Shared runtime, MPP, x402, and TAP suites on all four Python versions. Existing MPP Seller-subscription skips remain unchanged.
• Cancellation tests cover shared retrieval, all waiters leaving, resolver shutdown, and Python 3.14 late-failure diagnostics.

No payment workflow changes, live payments, or package release are included.