Miscellaneous code improvements.

[galenhuntington]

This PR addresses a handful of relatively straightforward issues flagged by a Claude Code review, plus a little more:

• An old bug where the last song is never randomly chosen is fixed. Actually, it was worse than that; the player crashes with divide by zero if there's only one song.
• The messy genericJumpToMatch is simplified. The pure (needed for the bespoke helper function) is floated out. Eventually how state is handled should be reformed, but for now this suffices.
• The README.md is moved to extra-doc-files where it belongs.
• The README clarifies use with Nix.
• The only use of non-decreasing indentation was removed above, so the extension is dropped.
• I got rid of pSafeHead and replaced its once use with something else technically safe. mpg123 sending invalid output is a remote concern so how it's handled matters little.
• The merge function is simplified by going through a Map.
• A minor thing, but the .mp3 extension is better checked for. When filename encodings are in better shape, we could replace with takeExtension.
• The spaces cache is dropped: it's both bad and pointless. Bad, because a P.take from a fixed big blank string would be far better. Pointless, because it's likely a minuscule optimization given all the other processing. Maybe this will be a Builder one day.
• I decided that hidden files and directories are better not included or traversed at all. So, there's no longer need to special case . and ... This is technically breaking so bumping minor version.
• A comment I added once which doesn't make sense to me now is removed.
• An encoding issue I stumbled on and filed is fixed; closes Unicode filenames on the command line cause a crash. #72.
• ncursesw is moved from extra-libraries to pkgconfig-depends.