Fixes the extension reporting "loaded 0 builders" right after the CUE parser binary is (re)installed.

findCacheDir picked the newest subdirectory of the cache root without excluding bin/ (the parser binary lives there) and ranked by directory mtime. Since extract.py rewrites metadata.json in place inside a stable content-hash dir — bumping the file's mtime but not the directory's — a freshly regenerated cache got shadowed by the newer bin/ dir, so no metadata loaded.

findCacheDir now skips bin/, requires a metadata.json, and ranks caches by that file's mtime.

Also ships the extract.py that materializes the full builder registry (including internal/ builders).