Bumps `@version` in `mix.exs` and the compiler's second carrier,
`@compiler_version` in `lib/statifier_blocks/compiler.ex`, to 0.31.0, and
the README install pin to `~> 0.31.0`. The compiler version moves on
every release because it is stamped into every compilation record, and
this release does move compiled chart bytes: a `core.on_event` handler
that names the outcome it finishes with compiles to a final of its own,
so a stored compiled chart recompiles to different bytes under the new
stamp.

Promotes the two `changelog.d/` fragments into a `## [0.31.0]` section
dated with the local date, grouped by heading and in fragment-name order
within each, and deletes them: sb-r6ln, sb-z0gz. Every bullet is carried
over byte for byte; the lead paragraph and the link reference are the
only prose written here.

A minor, not a patch, because a core block type takes a new public key
and what a named handler compiles to changes: `core.on_event` accepts an
optional `finish_as` key naming the outcome the handler finishes its
group with, which an enclosing composite can declare and route through
its derived `on_<name>` slot. Beside that, a slot written on a declaring
composite under the name the compiler reserves for that composite's
expansion members is reported as a compile finding instead of dropping
the children silently.

No tag, no publish: both are the operator's.

Refs: sb-q5a6