0.6.9: Finish the Stage 4 path revisit in the three remaining scripts

The 2026-07-08 path rewrite changed beets' displayed paths from the
transitional /music/... mount to real /data/music/Library/... paths.
dedup-library.sh was fixed in 0.6.8; three more scripts still assumed
the old prefix, each failing silently:

- replace-with-better.sh resolved every library match to a doubled
  nonexistent path, logged [stale-db], skipped the in-place replace,
  and then imported the staged FLAC as a NEW track via the
  leftover-import step. Net effect: the mp3-to-flac upgrade flow
  produced flac+mp3 twin pairs in the library (surfacing in the dedup
  queue) instead of replacing the MP3 in place.

- fix-track-metadata.py queried beets only by the legacy /music/...
  form, matched nothing, and silently skipped beet update/move after
  retagging.

- export-laptop-playlists.py filtered out every library track (no
  displayed path starts with /music/ anymore), producing empty
  exports.

All three now use the real displayed path and keep the /music/... form
only as a legacy fallback.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pipeline/lib/export-laptop-playlists.py

@@ -57,11 +57,12 @@ OUT_DIR = f"{_MUSIC_DATA_DIR}/playlists-laptop"
 # Absolute Windows path to the laptop's library root.
 LIBRARY_LAPTOP_ROOT = r"C:\Music\Library"
 
-# Beets stores paths with this prefix (the container-side mount of the
-# library); strip it to get the path relative to the library root. This is
-# still "/music/" through the migration's Stage 0-3 transitional mount;
-# revisit at Stage 4 once beets' directory: becomes MUSIC_DATA_DIR/Library.
-BEETS_LIBRARY_PREFIX = "/music/"
+# Prefixes beets may display library paths under, tried in order: the real
+# library dir (everything since the 2026-07-08 path rewrite) and the legacy
+# transitional mount (any stale pre-rewrite entry). Strip whichever matches
+# to get the path relative to the library root; when neither matches the
+# track is outside the library and is skipped.
+BEETS_LIBRARY_PREFIXES = (f"{_MUSIC_DATA_DIR}/Library/", "/music/")
 
 M3U_EXT = ".m3u"
 # Older formats from earlier iterations of this script — swept by reconcile.
@@ -157,9 +158,12 @@ def build_beets_index() -> dict[tuple[str, str], list[tuple[str, str, str, str]]
         if len(parts) != 4:
             continue
         aa, alb, ti, path = parts
-        if not path.startswith(BEETS_LIBRARY_PREFIX):
+        rel = next(
+            (path[len(p):] for p in BEETS_LIBRARY_PREFIXES if path.startswith(p)),
+            None,
+        )
+        if rel is None:
             continue
-        rel = path[len(BEETS_LIBRARY_PREFIX):]
         idx.setdefault((_normkey(alb), _normkey(ti)), []).append((aa, alb, ti, rel))
     return idx