Underscore filename schema, classical detection, NameToUnix post-processing

Pop schema:      TT_-_Artist_-_Title.ext
Classical schema: TT_-_Performer_-_Komponist_-_Werk[-_Orchester_Dirigent].ext
  triggered when albumartist ≠ track artist (pianist vs composer)

All spaces in names → underscores; separator _-_ between parts.
Missing parts (orchestra, conductor) are omitted.

models.py: added conductor/orchestra optional fields to TrackProposal.
executor.py: sanitize_dir_names() tries NameToUnix first, falls back to detox.
  Called after all renames in a directory are complete.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

models.py

@@ -61,6 +61,8 @@ class TrackProposal:
     disc_number: Optional[int]
     new_filename: Optional[str] = None   # only set when --rename is active
     mbid: Optional[str] = None
+    conductor: Optional[str] = None      # classical: Dirigent
+    orchestra: Optional[str] = None      # classical: Orchester / Ensemble
 
 
 @dataclass