Bumps @version and @compiler_version to 0.3.0, assembles the eighteen
changelog.d fragments into the 0.3.0 section and deletes them, and
points the README install snippet at ~> 0.3. The statifier floor stays
at ~> 2.2 and mix.lock does not move.

Full mix quality green (1,171 tests, 94.4%); mix docs clean; mix
hex.build produces statifier_blocks-0.3.0.tar with the expected files
and deps. Tag and publish are the operator's.

Refs: sb-dcn