feat(compat): add legacy purchase continuation coordinator

[bokelley]

Summary

• update the SDK to the signed AdCP 3.2.0-beta.4 schema bundle, generated models, exports, and conformance vectors
• add a durable legacy purchase continuation coordinator with exact 2.5.3, 3.0.18, and 3.1.15 source-schema validation
• add SQLite atomic claim, idempotent replay, fail-closed ambiguity, reconciliation, safe retention, and concurrent migration handling
• preserve beta.4 coordinator-input constraints in both Pydantic runtime validation and emitted JSON Schema

Safety

• stores only hashes of continuation bearer tokens
• binds principal, account, target, source version, observed payload, selection, pricing, and accepted losses before mutation
• retains unresolved operations and permits only explicitly safe reconciliation outcomes
• requires private SQLite file permissions and serializes schema migration across processes

Validation

• 6795 passed, 40 skipped, 9 deselected, 1 xfailed
• make lint
• make typecheck-all
• make validate-generated
• pre-commit Black, Ruff, mypy, Bandit, JSON, conflict, and private-key checks
• independent code and security expert reviews: no remaining merge blockers

Closes #1057