parseEpisode was handed the file's whole relative path, but the bare-number
pattern is anchored at the start of a name. Handed "Show/Season 3/06 -
Title.mkv" it could never match, so every file named that way was filed as a
standalone film at the library root instead of an episode.
Found by running a real scan against a real tree, not by reading the code:
the unit test on parseEpisode passed a bare file name, which is exactly the
input the call site was not providing. The unit test was right and the
caller was wrong, which is a class of bug a helper test cannot see.
parseEpisode now gets path.Base(rel), matching episodeTitle two lines below
it, so the function really is a function of a file name and its directory as
its signature and doc already claimed.
Both scan-level cases are added to the tree test, so the call site is now
covered and not just the helper. Verified red-green: reverting the call site
fails "bare leading number under a Season directory".
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