feat: implement durable legacy purchase continuation coordinator for AdCP 3.2

[bokelley]

Context

Protocol follow-up to adcp#6716 and adcp#6733.

AdCP 3.2 splits legacy compound media-buy operations across compact lifecycle tasks. The protocol now defines the compatibility coordinator input and the exact failure/atomicity contract, including products-only brief continuation and legacy inline-creative behavior.

Scope

Implement the Python SDK-local coordinator API: an idiomatic equivalent of continue_legacy_purchase(CompatibilityPurchaseCoordinatorInput).

• Consume the exact signed 3.2.0-beta.4 schema artifact once published.
• Provide a durable, pluggable continuation store.
• Bind continuations to principal, account, source protocol version, expiry, the full request payload, selected products/packages, and any listed-purchase context.
• Enforce atomic single-use claim, deterministic replay, fail-closed crash reconciliation, and the protocol's exact loss/error reporting.
• Support the products-only brief path and the reverse compact-seller → legacy-buyer facade where this SDK exposes server-side adapters.
• Keep operational controls distinct from commercial-term proposal successors.
• Run the shared products-only-brief-compatibility vectors from the beta.4 protocol bundle.

Acceptance criteria

• Coordinator API and durable-store interface are public and documented.
• Concurrent claims cannot purchase twice.
• Retry after success returns the deterministic prior result.
• Ambiguous/crashed claims fail closed and expose recovery guidance.
• Principal/account/version/expiry/payload mismatches are rejected.
• Shared beta.4 compatibility vectors pass.
• Migration guidance explains what remains application-owned.

Blocked until adcp#6733 merges and the signed 3.2.0-beta.4 artifact is available.