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/locales.py

@@ -29,8 +29,9 @@ _EN: dict[str, str] = {
     "pipeline.starting":             "Pipeline started",
     "pipeline.step1_name":           "Parse presets",
     "pipeline.step2_name":           "Compare presets",
-    "pipeline.step3_name":           "Download mods",
-    "pipeline.step4_name":           "Link mods",
+    "pipeline.step3_name":           "Migrate mod groups",
+    "pipeline.step4_name":           "Download mods",
+    "pipeline.step5_name":           "Link mods",
 
     # ── app.py dialogs ────────────────────────────────────────────────────────
     "app.dlg_presets_title":         "Not enough presets selected",
@@ -270,8 +271,9 @@ _VI: dict[str, str] = {
     "pipeline.starting":             "Pipeline đã bắt đầu",
     "pipeline.step1_name":           "Phân tích preset",
     "pipeline.step2_name":           "So sánh preset",
-    "pipeline.step3_name":           "Tải mod",
-    "pipeline.step4_name":           "Liên kết mod",
+    "pipeline.step3_name":           "Di chuyển nhóm mod",
+    "pipeline.step4_name":           "Tải mod",
+    "pipeline.step5_name":           "Liên kết mod",
 
     # ── app.py dialogs ────────────────────────────────────────────────────────
     "app.dlg_presets_title":         "Chưa chọn đủ preset",