refactor(dirvish): extract playlist filter and sanitize helpers

`cj/dired-create-playlist-from-marked' had its audio-file filtering and trailing-`.m3u' stripping inline among the dired marking, prompting, overwrite-loop, and file-write logic.  Lift each into its own pure helper:

- `cj/--playlist-filter-audio (files extensions)' returns only files whose extension matches one of EXTENSIONS (lowercase, no dot).  Case-insensitive on the file side.
- `cj/--playlist-sanitize-name (name)' strips a trailing `.m3u' suffix; embedded `.m3u' that isn't at the end is preserved.

Ten Normal/Boundary tests cover keep-only-audio, case-insensitivity, files-without-extension excluded, empty inputs, and the sanitize edge cases (bare name, embedded `.m3u', empty string, just `.m3u').

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