v0.14.0: a removal you make sticks

Round 43 made the merge report honest, and in doing so made its one real
limitation impossible to miss: every merge was a union, so a song removed from a
playlist on one machine was handed straight back by the other on the next sync,
and a track deleted from the library came back with it. A deletion you cannot
make stick is not a deletion.

The union was there for a real reason — two copies with no common ancestor
cannot tell "A added this" from "B removed it" — so the missing evidence is
written down instead of inferred. New lintunes/tombstones.py: a playlist keeps
track_events {track id: [when, add|remove]}, the library keeps deleted_tracks
{track id: when} in library_metadata.json, and a merge applies the newest event
per track across both copies. A removal beats a copy that merely still had the
song; a deliberate re-add afterwards beats the removal; a track nobody touched
still merges as a union, which stays the safe behavior where there is no
evidence either way.

Deliberately not "the newer copy wins wholesale": that one-liner silently drops
a song the other machine added while you were removing one, which
test_an_unrelated_addition_is_not_lost pins.

Events are recorded in the funnels that already exist — _set_track_ids diffs
before/after so a reorder records nothing, _remove_tracks stamps the library,
_restore_tracks clears it so Ctrl+Z takes the tombstone back — and pruned after
30 days at the save boundary.

Three edges worth naming:

* Track ids are never reused. The next id came from max(library.tracks), so
  deleting the highest-numbered track freed its id, and the next import would be
  dropped on sight by the dead id's own tombstone on every machine.
* library_metadata.json is merged before library.json, since it carries the
  record the library merge is filtered against and rglob order is not a plan.
* A merge applying a deletion never touches a music file — it drops the library
  entry only, and test_a_merge_never_touches_a_music_file fails the run if
  send_to_trash is so much as called. Applied removals grade WARNING and name
  the song.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y9ZEFi4qNJ39FMiBtiAxy2

tests/test_round43.py

@@ -242,7 +242,9 @@ class TestNamingTheSongs:
         body = "\n".join(summaries[0].lines)
         assert "the other copy did not have" in body
         assert "Pola — Abeille" in body
-        assert "delete it here too" in body
+        # Round 44: a removal now carries across on its own, so the note no
+        # longer tells you to delete it here by hand.
+        assert "Most likely you added it here" in body
 
     def test_the_window_caps_the_list_and_the_backup_holds_all_of_it(self, tmp_path):
         from lintunes.models import Track