Auto Decompiler Ranking

[PMD](https://pmd.github.io/) could be used to compare the output of different decompilers, and automatically pick which one to use based on the [complexity](http://pmd.sourceforge.net/pmd-4.3.0/rules/codesize.html) score. Other metrics can probably be used as well, but that would be the most basic.

Start by reviewing how bytecode-viewer currently selects and compares decompiler output; the issue names PMD complexity scoring as a possible basis but no files or entry points. Define which decompilers and metrics are supported, then verify that ranking consistently selects an output and handles comparison failures.

Written by the indexing model from the issue text.