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)

test_suite.py

@@ -2351,6 +2351,118 @@ test("_find_folder: missing meta.cpp silently skipped",
      _with_tmp(_test_ff_missing_meta_cpp_skipped))
 
 
+# ---------------------------------------------------------------------------
+# migrator — migrate_mod_groups
+# ---------------------------------------------------------------------------
+
+group("migrator — migrate_mod_groups")
+
+from arma_modlist_tools.migrator import migrate_mod_groups as _migrate
+
+
+def _make_mod(dl: Path, grp: str, folder: str, steam_id: str | None = None) -> Path:
+    """Create a minimal mod folder under downloads/group/folder."""
+    d = dl / grp / folder
+    d.mkdir(parents=True)
+    if steam_id:
+        (d / "meta.cpp").write_text(f"publishedid = {steam_id};\n", encoding="utf-8")
+    (d / "dummy.pbo").write_bytes(b"\x00" * 8)
+    return d
+
+
+def _test_migrate_already_correct():
+    comp = {"shared": {"mods": [{"name": "CBA_A3", "steam_id": "450814997"}]}, "unique": {}}
+    with tempfile.TemporaryDirectory() as d:
+        dl = Path(d) / "downloads"
+        _make_mod(dl, "shared", "@CBA_A3", "450814997")
+        result = _migrate(dl, None, comp)
+        assert_eq(result["moved"], 0)
+        assert_eq(result["skipped_correct"], 1)
+
+
+def _test_migrate_moves_by_steam_id():
+    comp = {"shared": {"mods": []}, "unique": {
+        "A_v1": {"mods": [{"name": "ACE3", "steam_id": "463939057"}]}
+    }}
+    with tempfile.TemporaryDirectory() as d:
+        dl = Path(d) / "downloads"
+        old = _make_mod(dl, "A", "@ACE3", "463939057")
+        result = _migrate(dl, None, comp)
+        assert_eq(result["moved"], 1)
+        assert not old.exists(), "old folder must be gone"
+        assert (dl / "A_v1" / "@ACE3").exists(), "new folder must exist"
+        assert (dl / "A_v1" / "@ACE3" / "dummy.pbo").exists(), "files preserved"
+
+
+def _test_migrate_moves_by_normalized_name():
+    """No meta.cpp — matching falls back to normalised folder name."""
+    comp = {"shared": {"mods": [{"name": "CBA_A3", "steam_id": None}]}, "unique": {
+        "A": {"mods": []}
+    }}
+    with tempfile.TemporaryDirectory() as d:
+        dl = Path(d) / "downloads"
+        _make_mod(dl, "A", "@CBA_A3", steam_id=None)
+        result = _migrate(dl, None, comp)
+        assert_eq(result["moved"], 1)
+        assert (dl / "shared" / "@CBA_A3").exists()
+
+
+def _test_migrate_skips_dest_exists():
+    comp = {"shared": {"mods": [{"name": "CBA_A3", "steam_id": "450814997"}]}, "unique": {}}
+    with tempfile.TemporaryDirectory() as d:
+        dl = Path(d) / "downloads"
+        _make_mod(dl, "A",      "@CBA_A3", "450814997")
+        _make_mod(dl, "shared", "@CBA_A3", "450814997")
+        result = _migrate(dl, None, comp)
+        assert_eq(result["moved"], 0)
+        assert_eq(result["skipped_dest_exists"], 1)
+
+
+def _test_migrate_skips_not_on_disk():
+    comp = {"shared": {"mods": [{"name": "CBA_A3", "steam_id": "450814997"}]}, "unique": {}}
+    with tempfile.TemporaryDirectory() as d:
+        dl = Path(d) / "downloads"
+        dl.mkdir()
+        result = _migrate(dl, None, comp)
+        assert_eq(result["skipped_not_found"], 1)
+        assert_eq(result["moved"], 0)
+
+
+def _test_migrate_removes_stale_junction():
+    from arma_modlist_tools.linker import create_junction, _is_junction
+    comp = {"shared": {"mods": [{"name": "ACE3", "steam_id": "463939057"}]}, "unique": {}}
+    with tempfile.TemporaryDirectory() as dl_d, \
+         tempfile.TemporaryDirectory() as arma_d:
+        dl   = Path(dl_d) / "downloads"
+        arma = Path(arma_d)
+        old  = _make_mod(dl, "A", "@ACE3", "463939057")
+        link = arma / "@ACE3"
+        create_junction(link, old)
+        assert _is_junction(link), "precondition: junction must exist"
+        result = _migrate(dl, arma, comp)
+        assert_eq(result["moved"], 1)
+        assert_eq(result["junction_removed"], 1)
+        assert not _is_junction(link), "stale junction must be removed"
+        assert (dl / "shared" / "@ACE3").exists()
+
+
+def _test_migrate_missing_downloads_dir():
+    comp = {"shared": {"mods": [{"name": "X", "steam_id": "1"}]}, "unique": {}}
+    with tempfile.TemporaryDirectory() as d:
+        result = _migrate(Path(d) / "nonexistent", None, comp)
+        assert_eq(result["moved"], 0)
+        assert_eq(result["skipped_not_found"], 1)
+
+
+test("migrator: already in correct group → no move",       _test_migrate_already_correct)
+test("migrator: moves mod by steam_id to new group",        _test_migrate_moves_by_steam_id)
+test("migrator: moves mod by normalized name (no meta.cpp)", _test_migrate_moves_by_normalized_name)
+test("migrator: skips when destination already exists",     _test_migrate_skips_dest_exists)
+test("migrator: skips mod not on disk",                     _test_migrate_skips_not_on_disk)
+test("migrator: removes stale junction before moving",      _test_migrate_removes_stale_junction)
+test("migrator: no-op when downloads dir missing",          _test_migrate_missing_downloads_dir)
+
+
 # ---------------------------------------------------------------------------
 # Summary
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