Files inside an Organize-by-Playlist folder were stuck with the library filename
(materializer hardcoded os.path.basename) — users wanted control over the naming,
e.g. a playlist-order prefix so a DAP plays them in order.
Add an opt-in FILENAME template "Playlist File Naming" (file_organization.templates
.playlist_item), tokens $position/$artist/$album/$track/$title. It is a filename,
not a path: validated to forbid "/" or "\" and to require $title, both in the
Settings UI (blocks save with a reason) and in core/playlists/item_naming.py, which
also fails safe at apply time — a bad/empty template falls back to the library
filename, so it can never produce a broken name. Default empty = current behavior.
Works for symlink AND copy modes (a symlink name is independent of its target).
Applied in _rebuild_one_from_db (the live reconcile/rebuild path), which has the
per-track metadata + playlist order; the pure FS materializer just gained an
optional dest_names override and is otherwise untouched. $position is zero-padded
to playlist width for correct sorting.
Tests: pure validate/render (slash + missing-title rejected, fallback, sanitize,
no-separator guarantee), FS-layer dest_names + collision disambiguation + back-
compat, and end-to-end through the DB rebuild (07->01 rename + empty-template
keeps library filename).
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