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When applying it in the plugins block, by id and version, this is the error:

Could not apply requested plugin [id: '****', version: '0.1.20'] as it does not provide a plugin with id '****'. This is caused by an incorrect plugin implementation. Please contact the plugin author(s).
> Plugin with id '****' not found.

…but the plugin is not incorrect (its very simple, works in other projects) - something about applying it to this project is awry. Have tried all kinds of combinations - publishing to Maven Central, Gradle Plugin portal, signing/not signing, stripping out anything else in the build script, stripping out guts of plugin class, etc to no avail.