events: tagged release will lock the experimental events package

[bilby91]

Per PRD §12.5 / design decision: the events surface is marked experimental until v1.0.0. We currently emit runtime.BuildEvent from the runtime layer; engine-level events (lifecycle phase start/end, container created, config resolved) per design/runtime.md §8 are not yet implemented.

Plan for v0.1.0

Decide one of:

1. Ship the engine event surface as designed (PRD §7.1 enumerates ~22 event types). Document as experimental.
2. Defer entirely until post-v0.1.0 to avoid locking in a half-finished surface that callers depend on.

Recommendation lean: defer. The runtime BuildEvent we already emit covers the docker-progress case; the spec-lifecycle events are nice-to-have but not blocking DAP cutover. Tracking here so we make the call deliberately.