fix: harden legacy purchase async settlement

[bokelley]

Summary

• harden legacy purchase settlement across direct responses, polling, A2A continuations, and webhooks
• preserve distinct client, seller-work, A2A, continuation, and delivery identities
• make deferred pause handoffs, callback publication, and mutation idempotency restart-safe and generation-fenced
• adopt the signed AdCP 3.2.0-beta.5 schema and compliance bundle
• enforce immutable, tenant-scoped webhook delivery bindings with durable retry state, advertised horizons, and permanent retired-ID tombstones
• keep synchronous terminal results silent on the task-webhook channel
• include the prerelease schema-sync parser fix already merged on main

Why

The compatibility surface had race windows where a mutation could commit while its callback, continuation, or terminal publication became stranded or ambiguous. AdCP 3.2 beta.5 now defines the cross-channel invariants; this PR implements them and fails closed when durable ownership or identity cannot be proven.

Validation

• npm run build:lib
• npm run format:check
• npm run typecheck
• npm run ci:codegen-strict
• npm run ci:schema-check
• npm run ci:docs-check
• npm run check:adopter-types
• npm run test:node:fast — 16,097 tests, 0 failures, 7 skipped on the pre-beta.5 review pass
• 89 focused webhook, security, and sync-silence tests
• 525 adjacent decisioning, TaskExecutor, and media-buy lifecycle tests
• repeated protocol, code-quality, security, and test expert review; all reported blockers addressed

[bokelley]

Merge-gate note: all product, schema, security, type, test, interop, and packaging checks are green, and all review threads are resolved. Ladon was rerun once and failed identically both times before review because GitHub refuses its full-diff request for PRs over 20,000 lines; it produced no findings file and posted no code finding. This PR has separately completed protocol, code, security, and test expert review. Using the maintainer override only for that documented review-infrastructure limitation.