compare: getRepositoryFilename crashes on path without '/'

`CompareMojo.getRepositoryFilename(Artifact)` (lines 328-331):

```java
String path = session.getRepositorySession().getLocalRepositoryManager().getPathForLocalArtifact(a);
return path.substring(path.lastIndexOf('/'));
```

`path.substring(path.lastIndexOf('/'))` throws `StringIndexOutOfBoundsException` if the local-repo layout returns a filename without a `/` separator (root-based `pom` layout in some configurations). Also a negative index propagates as a confusing runtime exception rather than a clean `MojoExecutionException`.

Suggested fix: guard `lastIndexOf('/')` and use `Path`-based extraction.

Start at CompareMojo.getRepositoryFilename(Artifact), lines 328-331, and inspect how the local repository manager supplies a path for a root-based pom layout. Verify handling for a filename without '/' and for a negative lookup, then run the project's existing tests; the issue does not name a specific test file. Done means these cases no longer produce a confusing StringIndexOutOfBoundsException.

Written by the indexing model from the issue text.