feat(acp): make hosted turns durable and observable

[RossHartmann]

Summary

• add a provider-neutral, locally signed publication worker with durable fences, idempotent receipts, editable progress, terminal replacement, bounded retries, and relay recovery
• preserve same-turn publication ordering while prioritizing terminal output and coalescing stale progress
• forward bounded canonical conversation context and wake quiet channels for due retries
• make cancellation author-scoped, classify terminal enrollment/auth failures without retry loops, and forward canonical relay context to child MCP sessions
• extend the SDK message builder with validated caller-owned durable tags

Why this is one change

The publication queue, ACP turn queue, process pool, error classification, and SDK fence tag form one turn-lifecycle state machine. Splitting them would create intermediate branches that compile but cannot preserve receipt ordering or terminal reconciliation. No provider name, credential, hosted service, or deployment policy is embedded here.

Validation

• cargo fmt --all -- --check
• cargo clippy -p buzz-acp -p buzz-sdk --all-targets --locked -- -D warnings
• cargo test -p buzz-acp local_publication --locked (13 passed)
• cargo test -p buzz-sdk message_with_extra_tags_preserves_durable_fence --locked (1 passed)
• seven added focused ACP tests covering author-scoped cancellation, MCP relay forwarding, terminal auth handling, event-id context normalization, and retry wakeups (7 passed)
• changed-diff scan found no Kiingo/provider identifier or common credential literal
• git diff --check

The commit is signed off under the DCO.