Add unified kernel status transition event type and Agent transition-event egress layer

Add the unified kernel status transition event and emit it ALONGSIDE the existing granular events. The granular events are NOT removed here - removal is deferred to BA-6853 (after the Manager starts consuming the unified event). This keeps the change additive and easy to roll back.

New event: KernelStatusTransitionAnycastEvent(kernel_id, from_status, to_status, reason, result, error_code, message). from_status/to_status use sokovan KernelStatus values; the event carries no phase/step payload; result is SUCCESS/FAILED only. Emit at the current lifecycle transition points, next to the existing granular anycast emission.

Does NOT depend on the handler framework (BA-6075) and does not touch create_kernel structure.

Success Criteria:
- [ ] KernelStatusTransitionAnycastEvent defined and wired to the anycast (at-least-once) egress
- [ ] Unified event emitted alongside existing granular events at lifecycle transition points
- [ ] Existing events/consumers unchanged (no regression)
- [ ] pants test passes for affected packages

JIRA Issue: BA-6851

Start by locating the current lifecycle transition points and the existing granular anycast emission in the affected packages. Trace the at-least-once egress path, then run the affected packages' pants tests; done means the unified event is emitted alongside existing events without changing current consumers.

Written by the indexing model from the issue text.