v0.1.1: perf timings + single-save tag writes + coalesced browser rebuild

Diagnosing the Ctrl+I save hang on the Debian machine (GUI thread blocks
past mutter's 5s check-alive):

- lintunes/perf.py: timed() context manager logging wall-clock ms at INFO
  on lintunes.perf — instruments tag/artwork saves, file moves,
  edit_track(s)_fields, the browser rebuild, smart recompute, and the
  debounced JSON flush, so the slow machine can tell us which stage
  eats the time (journalctl --user or a terminal run).
- tagging.write_tags: ONE parse + ONE save per edit. grouping/
  compilation/bpm ride the main save via registered Easy keys
  (GRP1/TCMP/TBPM on EasyID3, cpil on EasyMP4) instead of
  _write_extra_tags re-parsing and re-saving the audio file.
- LibraryView: browser rebuilds coalesce through a 0ms single-shot
  timer — an N-track Get Info edit rebuilds the genre/artist/album
  cascade once instead of N times over the whole library.

Moving the writes off the GUI thread is deliberately deferred until the
Debian timings say which stage dominates.

lintunes/gui/library_view.py

@@ -8,6 +8,7 @@ from PyQt6.QtCore import Qt, QTimer, pyqtSignal
 
 from lintunes.gui.table_settings import TableSettingsMixin
 from lintunes.gui.track_table import TrackTableView
+from lintunes.perf import timed
 
 
 _ARTICLE_RE = re.compile(r"^the\s+", re.IGNORECASE)
@@ -162,6 +163,13 @@ class LibraryView(TableSettingsMixin, QWidget):
 
         manager.track_updated.connect(self.table.model_.refresh_track)
         manager.track_fields_edited.connect(self._on_track_fields_edited)
+        # Coalesces browser rebuilds: a multi-track edit emits
+        # track_fields_edited once per track, and rebuilding the whole
+        # genre/artist/album cascade per signal is O(edited × library).
+        self._browser_rebuild_timer = QTimer(self)
+        self._browser_rebuild_timer.setSingleShot(True)
+        self._browser_rebuild_timer.setInterval(0)
+        self._browser_rebuild_timer.timeout.connect(self._rebuild_browser)
 
         self.reload()
 
@@ -295,8 +303,17 @@ class LibraryView(TableSettingsMixin, QWidget):
         rebuild the lists and re-filter so the renamed value sorts to its new
         spot and clicking it still finds its tracks (the plain row repaint from
         track_updated leaves the browser stale). Non-browser edits (name, bpm…)
-        are handled by the proxy's live re-sort, so they need nothing here."""
+        are handled by the proxy's live re-sort, so they need nothing here.
+
+        The rebuild is deferred to a 0 ms single-shot timer so an N-track edit
+        (which fires this once per track) rebuilds the browser once, not N
+        times."""
         if _BROWSER_FIELDS.intersection(changed_fields):
+            self._browser_rebuild_timer.start()
+
+    def _rebuild_browser(self):
+        with timed("library browser rebuild (%d tracks)",
+                   len(self._all_tracks)):
             self._populate_genres()
             self._populate_artists()
             self._populate_albums()