Release prep only: the tag, the Hex publish and the GitHub release stay
the operator's. A minor, not a patch: the ChildStarter callback spec
changed - `start_child/5`'s first argument is now named
`parent_execution_id` - so the published contract moved, even though the
argument is positional and an existing starter module compiles unchanged.

- mix.exs: @version 0.9.1 -> 0.10.0
- CHANGELOG.md: new [0.10.0] 2026-09-12 section (the local date), the
  sob-mh3 fragment's two Changed bullets carried over byte for byte -
  they are single long lines in the fragment and stay that way here, since
  rewrapping is not a byte-for-byte carry; no lead paragraph, since the
  bullets say the whole of it
- changelog.d/sob-mh3.md: deleted, promoted into the section above; it was
  the only fragment the directory held
- README.md: install pin `~> 0.9` -> `~> 0.10`, since the old constraint
  no longer admits the version being released

Gate: `mix quality` green on this tree (format, compile, deps, credo,
244 tests / 96.4% coverage, dialyzer) under the repo gate lock and
machine slot-1, which were held across the commit.

Refs: sob-e30