feat: add migrate step to move mod folders between groups on preset change

Before step_fetch, scan all downloads/ subdirs and move any mod that
comparison.json now assigns to a different group. Matching uses steam_id
(via meta.cpp publishedid) first, normalized name as fallback.

Stale junctions in arma_dir are removed before the folder move so
step_link can re-create them pointing to the new location.

- New arma_modlist_tools/migrator.py: migrate_mod_groups()
- run.py: step_migrate(), --skip-migrate flag, wired into dispatch loop
- gui/app.py: step_migrate inserted as Step 3/5 between compare and fetch
- gui/locales.py: add step3/4/5 names (en + vi), renumber old 3->4, 4->5
- test_suite.py: 7 new migrator tests (158 total, 0 failed)

run.py

@@ -78,6 +78,22 @@ def step_compare(cfg) -> None:
     print(f"  -> {cfg.comparison}")
 
 
+def step_migrate(cfg) -> None:
+    if not cfg.comparison.exists():
+        print(f"  NOTE: {cfg.comparison} not found — skipping migration.")
+        return
+    if not cfg.downloads.is_dir():
+        print(f"  NOTE: downloads dir missing ({cfg.downloads}) — nothing to migrate.")
+        return
+    from arma_modlist_tools.migrator import migrate_mod_groups
+    comparison = json.loads(cfg.comparison.read_text(encoding="utf-8"))
+    arma_dir   = cfg.arma_dir if cfg.arma_dir.is_dir() else None
+    result     = migrate_mod_groups(cfg.downloads, arma_dir, comparison)
+    if result["errors"]:
+        for name, err in result["errors"].items():
+            print(f"  ERROR  {name}: {err}")
+
+
 def step_fetch(cfg) -> None:
     if not cfg.comparison.exists():
         print(f"  ERROR: {cfg.comparison} not found. Run parse + compare first.")
@@ -178,6 +194,7 @@ def main() -> None:
     parser = argparse.ArgumentParser(description="Run the full mod management pipeline.")
     parser.add_argument("--skip-parse",   action="store_true")
     parser.add_argument("--skip-compare", action="store_true")
+    parser.add_argument("--skip-migrate", action="store_true")
     parser.add_argument("--skip-fetch",   action="store_true")
     parser.add_argument("--skip-link",    action="store_true")
     parser.add_argument("--group", "-g",  metavar="GROUP",
@@ -187,13 +204,15 @@ def main() -> None:
     steps = [
         (not args.skip_parse,   "Parse presets"),
         (not args.skip_compare, "Compare presets"),
+        (not args.skip_migrate, "Migrate mods"),
         (not args.skip_fetch,   "Fetch mods"),
         (not args.skip_link,    "Link mods"),
     ]
     active_count = sum(1 for run, _ in steps if run)
     step_num = 0
 
-    if args.skip_parse and args.skip_compare and args.skip_fetch and args.skip_link:
+    if (args.skip_parse and args.skip_compare and args.skip_migrate
+            and args.skip_fetch and args.skip_link):
         print("All steps skipped — nothing to do.")
         sys.exit(0)
 
@@ -201,6 +220,7 @@ def main() -> None:
         (cfg,),
         (cfg,),
         (cfg,),
+        (cfg,),
         None,  # handled separately
     ]):
         if not run:
@@ -221,6 +241,8 @@ def main() -> None:
             step_parse(cfg)
         elif name == "Compare presets":
             step_compare(cfg)
+        elif name == "Migrate mods":
+            step_migrate(cfg)
         elif name == "Fetch mods":
             step_fetch(cfg)