feat(protocol)!: add AdCP 3.2 beta.5 parity

[bokelley]

Summary

• pin and package the AdCP 3.2.0-beta.5 schema bundle and regenerate Python models/versioned exports
• implement beta.5 async identity, terminal convergence, webhook idempotency, retention, and trusted receiver scoping invariants
• align TaskHandoff/WorkflowHandoff admission with the external durable-publisher contract and preserve canonical task identities
• update decisioning rights/creative surfaces, migration guidance, examples, and conformance coverage

Validation

• full suite before final targeted refinements: 6909 passed, 40 skipped, 9 deselected, 1 xfailed
• final dispatch/creative/rights focused suite: 252 passed
• lint, full mypy, strict adopter type checks, generated-code validation, and diff hygiene pass
• wheel and sdist package checks include the complete beta.5 schema bundle
• code, protocol, and security expert reviews approved with no remaining blockers

BREAKING CHANGE: Webhook receiver configuration now requires trusted receiver and publisher scopes, webhook deduplication uses explicit claim/acknowledge/release ownership, beta.5 decisioning Protocols align with their wire response arms, and hand-written Submitted responses are rejected in favor of registry-backed handoffs. See docs/webhooks/migration-from-fragmented-senders.md and SCHEMA_DELTAS.md for migration guidance.