fix(buzz-acp): post a channel notice on cancel+merge steer fallback (WO #301)

[Theremoteaidoc]

Closes block#301
Closes Theremoteaidoc/remote-aid-medical#301

When an adapter lacks _session/steering and has no goose active_run_id,
buzz-acp already fell back to cancel+merge. That path was silent: only a
debug log (agent_returned (cancelled)). A follow-up mention looked like a
model failure.

This PR posts one channel-visible kind:9 line when that fallback fires, and
pins that the cancelled prompt is merged into the next FlushBatch and
rendered into the agent prompt.

Channel line (example for seat Cursor):

Cursor's in-flight turn was cancelled by an incoming message.

Re-delivery: already implemented via queue.requeue_as_cancelled +
flush_next merge. Verified end-to-end (withhold → release → requeue →
flush → format_prompt). No production gap found; new tests pin it.

Native Success path (Ok(SteerAck::Success)) is unchanged: drop withheld,
no fallback, no notice.

Follow-ups (not in this PR)

• Implement _session/steering in cursor_acp.py, opencode_acp.py,
  pi_acp.py (non-cancelling steer).
• Convert the harness from cancel-then-merge to queue-instead-of-cancel.

files-touched:

• crates/buzz-acp/src/lib.rs
• crates/buzz-acp/src/queue.rs

Evidence

$ cargo test -p buzz-acp steer
     Running unittests src/lib.rs (target/debug/deps/buzz_acp-cffac62f1d39cfb8)

running 28 tests
test acp::tests::acp_steer_failed_outcome_acks_outcome_rejected ... ok
test acp::tests::acp_steer_injected_does_not_renew_hard_deadline ... ok
test acp::tests::acp_steer_missing_outcome_acks_outcome_rejected_and_never_drops_event ... ok
test acp::tests::acp_steer_request_omits_expected_run_id_and_carries_session_and_prompt ... ok
test acp::tests::acp_steer_started_new_turn_acks_success_without_renewing_hard_deadline ... ok
test acp::tests::initialize_leaves_steering_unsupported_when_explicitly_false ... ok
test acp::tests::initialize_leaves_steering_unsupported_when_meta_absent ... ok
test acp::tests::initialize_records_steering_supported_when_advertised ... ok
test acp::tests::native_steer_with_active_run_id_routes_response_to_ack ... ok
test acp::tests::native_steer_with_no_active_run_id_acks_expected_run_id_missing ... ok
test acp::tests::steer_success_does_not_renew_hard_deadline_and_hard_timeout_fires ... ok
test acp::tests::steer_writes_nothing_when_no_run_id_and_capability_absent ... ok
test author_gate_tests::test_owner_only_admits_owner_and_sibling_to_steer ... ok
test author_gate_tests::test_owner_only_rejects_stranger_so_no_steer ... ok
test cancel_merge_fallback_notice_tests::steer_ack_success_disposition_is_unchanged ... ok
test config::tests::test_multiple_event_handling_default_is_steer ... ok
test config::tests::test_validate_steer_requires_queue_dedup ... ok
test pool::tests::test_send_prompt_result_clears_steer_rx_on_early_return ... ok
test pool::tests::test_send_prompt_result_is_noop_when_steer_rx_already_consumed ... ok
test queue::tests::test_format_prompt_no_reason_defaults_to_steer_framing ... ok
test queue::tests::test_format_prompt_steer_framing ... ok
test queue::tests::test_format_prompt_steer_framing_multi_event ... ok
test queue::tests::test_native_steer_bulk_release_preserves_fifo ... ok
test queue::tests::test_native_steer_earlier_events_flush_during_ack_window ... ok
test queue::tests::test_native_steer_expiry_recovers_withheld ... ok
test queue::tests::test_native_steer_withhold_only_channel_not_flushable ... ok
test queue::tests::test_steer_cross_thread_reply_targets_steering_message ... ok
test queue::tests::test_steer_end_to_end_queue_to_rendered_prompt ... ok

test result: ok. 28 passed; 0 failed; 0 ignored; 0 measured; 682 filtered out; finished in 9.36s

$ cargo test -p buzz-acp error_outcome_emission_tests
     Running unittests src/lib.rs (target/debug/deps/buzz_acp-cffac62f1d39cfb8)

running 22 tests
test error_outcome_emission_tests::agent_exited_emits_exactly_one_feed_event ... ok
test error_outcome_emission_tests::application_error_emits_exactly_one_feed_event ... ok
test error_outcome_emission_tests::auth_error_dead_letters_immediately_without_requeueing ... ok
test error_outcome_emission_tests::cancel_drain_timeout_dropped_stop_batch_none_same_neutral_payload ... ok
test error_outcome_emission_tests::cancel_drain_timeout_emits_exactly_one_feed_event ... ok
test error_outcome_emission_tests::cancel_drain_timeout_requeues_batch_and_does_not_return_agent ... ok
test error_outcome_emission_tests::cancelled_prompt_redelivers_original_event_to_agent_prompt ... ok
test error_outcome_emission_tests::hard_timeout_emits_exactly_one_feed_event ... ok
test error_outcome_emission_tests::hard_timeout_not_requeued_idle_timeout_is_requeued ... ok
test error_outcome_emission_tests::hard_timeout_recently_active_budget_exhausted_reports_dead_lettered ... ok
test error_outcome_emission_tests::hard_timeout_recently_active_requeue_success_reports_requeued_for_retry ... ok
test error_outcome_emission_tests::hard_timeout_recently_active_requeues_batch ... ok
test error_outcome_emission_tests::idle_timeout_emits_exactly_one_feed_event ... ok
test error_outcome_emission_tests::is_auth_error_matches_401_message ... ok
test error_outcome_emission_tests::is_auth_error_matches_reauthenticate_message ... ok
test error_outcome_emission_tests::is_auth_error_rejects_other_agent_error_message ... ok
test error_outcome_emission_tests::is_auth_error_rejects_transport_errors ... ok
test error_outcome_emission_tests::non_auth_application_error_is_requeued ... ok
test error_outcome_emission_tests::normalizes_agent_name_from_initialize_result ... ok
test error_outcome_emission_tests::panic_event_retains_task_turn_id ... ok
test error_outcome_emission_tests::timeout_outcome_labels_differ ... ok
test error_outcome_emission_tests::transport_error_emits_exactly_one_feed_event ... ok

test result: ok. 22 passed; 0 failed; 0 ignored; 0 measured; 688 filtered out; finished in 0.03s

$ cargo test -p buzz-acp cancel
     Running unittests src/lib.rs (target/debug/deps/buzz_acp-cffac62f1d39cfb8)

running 26 tests
test acp::tests::cancel_with_cleanup_grace_maps_expiry_to_cancel_drain_timeout ... ok
test acp::tests::permission_cancelled_response_preserves_id_type ... ok
test acp::tests::permission_request_with_no_options_is_cancelled ... ok
test acp::tests::permission_request_without_reject_once_is_cancelled ... ok
test acp::tests::permission_response_cancelled_format ... ok
test acp::tests::session_cancel_notification_has_session_id_in_params ... ok
test cancel_merge_fallback_notice_tests::cancel_merge_fallback_notice_names_seat_and_incoming_cancel ... ok
test cancel_merge_fallback_notice_tests::cancel_merge_fallback_posts_exactly_one_channel_notice_per_fallback_event ... ok
test cancel_merge_fallback_notice_tests::steer_ack_success_disposition_is_unchanged ... ok
test error_outcome_emission_tests::cancel_drain_timeout_dropped_stop_batch_none_same_neutral_payload ... ok
test error_outcome_emission_tests::cancel_drain_timeout_emits_exactly_one_feed_event ... ok
test error_outcome_emission_tests::cancel_drain_timeout_requeues_batch_and_does_not_return_agent ... ok
test error_outcome_emission_tests::cancelled_prompt_redelivers_original_event_to_agent_prompt ... ok
test pool::tests::test_cancel_after_natural_completion_preserves_channel_state ... ok
test pool::tests::test_classify_control_cancel_failure_crosses_error_outcome_and_batch_fate ... ok
test pool::tests::test_publish_agent_turn_metric_cancelled_stop_reason ... ok
test pool::tests::test_requeue_cancelled_batch_maps_control_signal_to_cancel_reason ... ok
test pool_lifecycle::tests::cancelled_wake_enters_failed_and_can_retry ... ok
test queue::tests::test_cancel_merge_fallback_withhold_release_redelivers_to_prompt ... ok
test queue::tests::test_double_cancel_latest_reason_wins ... ok
test queue::tests::test_double_cancel_preserves_all_events ... ok
test queue::tests::test_drain_channel_clears_cancelled_batches ... ok
test queue::tests::test_has_flushable_work_with_cancelled_only ... ok
test queue::tests::test_requeue_as_cancelled_merges_in_flush_next ... ok
test queue::tests::test_requeue_as_cancelled_no_new_events_fallback ... ok
test queue::tests::test_requeue_as_cancelled_propagates_reason ... ok

test result: ok. 26 passed; 0 failed; 0 ignored; 0 measured; 684 filtered out; finished in 3.38s