fix: replace Unicode checkmark with ASCII + for Windows encoding (#535)

Windows terminals using cp1251/cp1252 crash on the Unicode ✓ (U+2713)
in progress output. Replace with ASCII + in convo_miner.py and
split_mega_files.py.

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

mempalace/convo_miner.py

@@ -471,7 +471,7 @@ def mine_convos(
             room_counts[r] += n
 
         total_drawers += drawers_added
-        print(f"  ✓ [{i:4}/{len(files)}] {filepath.name[:50]:50} +{drawers_added}")
+        print(f"  + [{i:4}/{len(files)}] {filepath.name[:50]:50} +{drawers_added}")
 
     print(f"\n{'=' * 55}")
     print("  Done.")