test(plugin): cover composer/installers beyond a fresh install

A single install leaves the installer contract half tested: it never reaches
`update`, never runs over an already-installed tree, and never reaches the
plugin's own `uninstall()` override — the one chaining onto the promise
`LibraryInstaller::uninstall` returns. Nor does it touch the configuration
surface real projects use, `installer-paths` and `installer-name`, which the
plugin reads back through the package proxy.

All three now compare command output as well as the resulting tree against
upstream Composer. Progress bar frames are dropped from that comparison:
Shirabe renders them differently for every install, including projects with
no plugin at all, so they say nothing about the plugin under test.

With those covered the plugin joins the verified list in the README.

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