sorter: write audit + low-confidence doc beside the library, never inside it

The library may be a seeded torrent whose file set is frozen; the
sorter_applied.jsonl / sorter_low_confidence.txt bookkeeping now goes
to <library>/../_logs/ by default. Explicit --audit still wins.

sorter.py

@@ -15,7 +15,9 @@
 
 Flags:
     --catalog FILE   default data/library_catalog.json
-    --audit FILE     default <library>/sorter_applied.jsonl
+    --audit FILE     default <library>/../_logs/sorter_applied.jsonl
+                     (bookkeeping stays OUT of the library, which may be a
+                     seeded torrent; --audit still overrides)
 """
 
 from __future__ import annotations
@@ -85,7 +87,11 @@ def main() -> None:
     catalog = load_catalog(args.catalog, dropped=dropped)
     if dropped:
         log.info("Dropped systems: %s", ", ".join(sorted(dropped)))
-    audit = args.audit or (args.library / "sorter_applied.jsonl")
+    # Bookkeeping must never land inside the library itself (it may be a
+    # seeded torrent whose file set is frozen): default to a _logs folder
+    # beside it. An explicit --audit overrides this.
+    audit = args.audit or (args.library.parent / "_logs" / "sorter_applied.jsonl")
+    log.info("Audit file: %s", audit)
 
     provenance = None
     if args.provenance: