Bumps @version to 0.3.0 and assembles the single unreleased
changelog.d fragment (sp-e50) into a dated 0.3.0 section, deleting it
in the same commit as changelog.d/README.md prescribes.

Minor rather than patch: the async-invocation seam adds public surface
(a `:pending` dispatch answer, `Driver.done_invocation/5` and
`failed_invocation/5`, and an event-builder arm on `Runs.step/5`)
while every existing call keeps working. For a 0.x package that
additive surface is the minor arm.

Prep only: no tag, no publish, no hex task - those stay the
operator's.

Refs: sp-djo