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)

gui/app.py

@@ -151,7 +151,7 @@ class ArmaModManagerApp(ctk.CTk):
             # run.py calls fix_console_encoding() at import time, which needs
             # the real sys.stdout.buffer. Import it before we redirect stdout.
             try:
-                from run import step_fetch, step_link
+                from run import step_migrate, step_fetch, step_link
             except Exception as _import_err:
                 self.after(0, lambda: self.post_log(
                     f"\n✗  Failed to load pipeline: {_import_err}\n"
@@ -165,7 +165,7 @@ class ArmaModManagerApp(ctk.CTk):
                 from arma_modlist_tools.compare import compare_presets
 
                 # Step 1 — Parse selected presets
-                _hdr("Step 1 / 4", t("pipeline.step1_name"))
+                _hdr("Step 1 / 5", t("pipeline.step1_name"))
                 cfg.modlist_json.mkdir(exist_ok=True)
                 presets = []
                 for fp in sorted(cfg.modlist_html.glob("*.html")):
@@ -182,7 +182,7 @@ class ArmaModManagerApp(ctk.CTk):
                     presets.append(preset)
 
                 # Step 2 — Compare
-                _hdr("Step 2 / 4", t("pipeline.step2_name"))
+                _hdr("Step 2 / 5", t("pipeline.step2_name"))
                 result = compare_presets(*presets)
                 cfg.comparison.write_text(
                     json.dumps(result, indent=2, ensure_ascii=False),
@@ -193,12 +193,16 @@ class ArmaModManagerApp(ctk.CTk):
                 print(f"  Shared: {result['shared']['mod_count']}  |  "
                       f"Unique: {total_unique}")
 
-                # Step 3 — Fetch
-                _hdr("Step 3 / 4", t("pipeline.step3_name"))
+                # Step 3 — Migrate
+                _hdr("Step 3 / 5", t("pipeline.step3_name"))
+                step_migrate(cfg)
+
+                # Step 4 — Fetch
+                _hdr("Step 4 / 5", t("pipeline.step4_name"))
                 step_fetch(cfg)
 
-                # Step 4 — Link
-                _hdr("Step 4 / 4", t("pipeline.step4_name"))
+                # Step 5 — Link
+                _hdr("Step 5 / 5", t("pipeline.step5_name"))
                 groups = (
                     sorted(p.name for p in cfg.downloads.iterdir() if p.is_dir())
                     if cfg.downloads.is_dir() else []