Round 17: task batch + cruft sweep (ratings, art download, MPRIS/exit/BT fixes)

Features:
- Rating hover dots: hovering a rating cell shows 5 clickable slots
  (RatingDelegate); click slot k sets k stars, clicking the current count
  clears. Undoable, library-only (ratings never rewrite music files).
- Search bar moved into the Library header strip, right of the Library
  button, so the tracklist top aligns with the playlist tree.
- Right-click "Download Album Art…": iTunes Search API (no key), off-thread
  fetch, preview/confirm dialog with Next Result, embeds via write_artwork
  + size refresh, invalidates the MPRIS art cache.

Fixes:
- MPRIS media keys: PropertiesChanged sent invalidated_properties as "av"
  instead of "as", so gsd-media-keys dropped it and never MRU-bumped
  lintunes (why the play/pause key kept waking stale players). Now an
  explicit QDBusArgument string array; loopback-verified sa{sv}as.
- Exit segfault: ordered Player.shutdown() (stop, clear source, detach
  buffer/audio outputs) from closeEvent/aboutToQuit; scripted run exits 0.
- BT zero-volume after pause/resume: volume re-applied on resume, device
  swap, and BufferedMedia (needs verify on the affected machine).

Cruft sweep:
- Tag writes filtered to EDITABLE_FIELDS; failures logged + surfaced in
  the status bar (was a swallowed print).
- O(n²) import fixed (location index + cached max track id); O(1)
  refresh/reveal via TrackTableModel row index.
- lastfm: scrobble-queue thread lock, login prefs write marshalled to the
  GUI thread, JSON/raise_for_status order fixed.
- Shared read_json/write_json; TableSettingsMixin dedupes view settings.

TASKS.md rewritten as a resumable board; tests in tests/test_round17.py
(251 total pass).

lintunes/library_manager.py

@@ -1,9 +1,12 @@
+import logging
 import secrets
 from datetime import datetime, timezone
 from pathlib import Path
 
 from PyQt6.QtCore import QObject, QTimer, pyqtSignal
 
+log = logging.getLogger(__name__)
+
 from lintunes import smart, tagging
 from lintunes.models import Library, Playlist, PlaylistType
 from lintunes.storage import json_storage
@@ -32,6 +35,7 @@ class LibraryManager(QObject):
     track_fields_edited = pyqtSignal(int, list)  # track_id, changed field names
     library_reloaded = pyqtSignal()            # library re-read from disk (sync)
     conflict_resolved = pyqtSignal(list)        # list[ConflictSummary] just merged
+    tag_write_failed = pyqtSignal(str, str)     # track name, error text
 
     def __init__(self, library: Library, data_dir: Path, parent=None):
         super().__init__(parent)
@@ -65,10 +69,12 @@ class LibraryManager(QObject):
         self._own_sigs: dict[str, tuple | None] = {}
         self.snapshot_sync_state()
 
+        self._max_track_id = max(self.library.tracks.keys(), default=0)
+
     # ---- id generation ----
 
     def new_track_id(self) -> int:
-        return max(self.library.tracks.keys(), default=0) + 1
+        return self._max_track_id + 1
 
     def new_persistent_id(self) -> str:
         while True:
@@ -312,6 +318,7 @@ class LibraryManager(QObject):
         if not track.persistent_id:
             track.persistent_id = secrets.token_hex(8).upper()
         self.library.tracks[track.track_id] = track
+        self._max_track_id = max(self._max_track_id, track.track_id)
         self._dirty_tracks = True
         self._dirty_metadata = True
         self._schedule_save()
@@ -403,13 +410,18 @@ class LibraryManager(QObject):
                           for tid, new, _old in changes]))
 
     def _write_track_tags(self, track, field_map: dict) -> bool:
-        if not track.location:
+        # Library-only fields (rating, size, start/stop times) have no tag
+        # representation; don't rewrite the audio file for them.
+        writable = {k: v for k, v in field_map.items()
+                    if k in tagging.EDITABLE_FIELDS}
+        if not writable or not track.location:
             return True
         try:
-            tagging.write_tags(track.location, field_map)
+            tagging.write_tags(track.location, writable)
             return True
         except Exception as e:
-            print(f"Could not write tags to {track.location}: {e}")
+            log.warning("Could not write tags to %s: %s", track.location, e)
+            self.tag_write_failed.emit(track.name or track.location, str(e))
             return False
 
     def _apply_track_fields(self, track_id: int, field_map: dict):
@@ -653,6 +665,8 @@ class LibraryManager(QObject):
             self.library.import_date = disk.import_date
             self.library.library_settings = disk.library_settings
 
+        self._max_track_id = max(self.library.tracks.keys(), default=0)
+
         self._recomputing = True
         try:
             self.recompute_all_smart()