Promotes the six changelog.d fragments into a `## [0.9.0]` section:
sb-flae, sb-2epo, sb-pok4, sb-ht79, sb-6b1 and sb-lpdt, grouped
Added 9 / Changed 2 / Fixed 4. Every fragment bullet is carried over
byte for byte; the prose written here is the lead paragraph and the
`[0.9.0]` link reference.

sb-ae8o shipped no fragment and needs none. It tidied records and
comments, which the changelog.d README excludes by name as work a
caller of the public API cannot see. sb-o6ww closed without a change.

Bumps the version to 0.9.0 in mix.exs, in the compiler's
`@compiler_version`, and in the README's install pin. The compiler
identity assertion reads the version relationally, so no test moves.

Not tagged and not published: `mix hex.build` was run as proof and its
tarball deleted.

Refs: sb-q6gg