Promotes the seven changelog.d fragments into a `## [0.6.0]` section:
sb-2mxa, sb-3pv4, sb-dbqq, sb-ehqn, sb-lti6, sb-ukgu and sb-vamn,
grouped Added 4 / Changed 6 / Removed 2. Every bullet is carried over
byte for byte; the only prose written here is the lead paragraph and
the `[0.6.0]` link reference. The fragments are deleted, leaving
changelog.d with its README.

Bumps the version to 0.6.0 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.

Fixes the wording of one 0.5.0 bullet, which described the band tokens
in a clause with no predicate and stopped mid-sentence.

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

Refs: sb-fwi0