fix: keep the mame clone map across runs

A clone group is only visible while both copies are on disk, and the
run then deletes the clone, so writing only what this run saw erased
every mapping an earlier run had recorded. One real run took the map
from 69 entries to 1, and the canonical zips silently stopped answering
to the names they stand in for.

The map is merged now, the 68 lost entries are restored, and the one
whose canonical file is no longer in the collection is dropped.

scripts/dedup.py

@@ -228,6 +228,21 @@ def deduplicate(bios_dir: str, dry_run: bool = False) -> dict:
     # Write MAME clone mapping
     if mame_clones:
         clone_path = "_mame_clones.json"
+        # A group is only visible while both copies are on disk, and this run
+        # has just deleted the clone. Writing only what was seen this time
+        # therefore erases every mapping an earlier run recorded, and the
+        # canonical zip stops answering to the names it stands in for. The
+        # older entries stay valid: their canonical file is still here.
+        if not dry_run:
+            try:
+                with open(clone_path) as f:
+                    previous = json.load(f)
+            except (FileNotFoundError, json.JSONDecodeError):
+                previous = {}
+            if isinstance(previous, dict):
+                merged = dict(previous)
+                merged.update(mame_clones)
+                mame_clones = merged
         if dry_run:
             print(f"\nWould write MAME clone map: {clone_path}")
             print(