Ship a public-domain Für Elise (keys) feedpak as starter content so a fresh
install isn't an empty library. server._seed_builtin_starter_content() copies
bundled packs into DLC_DIR/starter/ exactly once, guarded by a marker in
CONFIG_DIR — unlike the always-reseeding diagnostic seed, a user who deletes
the starter song does not get it back. `starter/` is deliberately outside the
diagnostics/tutorials library carve-out so the song surfaces as a normal
library entry.
Extract the shared symlink-safe, mtime-aware copy loop into
_copy_builtin_packs() and route both the diagnostic and starter seeds through
it (diagnostic behavior unchanged; existing tests green).
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
