Promotes the one changelog.d fragment into a `## [0.12.0]` section:
sb-uag7, grouped Added 6. Every fragment bullet is carried over byte for
byte; the prose written here is the lead paragraph and the `[0.12.0]`
link reference.

Minor, not patch: the fragment is Added-only, so the release is the
drafts shelf and the placeholder marker over 0.11.0 with nothing changed
or removed.

Bumps the version to 0.12.0 in mix.exs, in the compiler's
`@compiler_version`, and in the README's install pin - the three
carriers the release recipe in `.claude/wurk.json` and the extension in
`.claude/wurk/release.md` name between them. The compiler identity
assertion reads the version relationally, so no test moves.

Not tagged and not published, and no hex task was run: the operator tags
and publishes after the campaign wrap. The full gate is the proof in its
place.

Refs: sb-gywd