sorter: drop non-warhammer game systems (default: Middle-Earth)

The collection is warhammer-only; everything else stays in _unsorted
even when names match the catalog. The catalog snapshot keeps all
systems - the filter is a sorter default (DROPPED_SYSTEMS, applied in
load_catalog and after keyword classification), adjustable with
--drop-system/--keep-system.

tests/test_sorter_units.py

@@ -30,6 +30,9 @@ CATALOG = {
         {"name": "Dark Reapers", "system": "Warhammer 40,000",
          "group": "Xenos Armies", "faction": "Aeldari",
          "class": "Aspect Warriors", "aliases": []},
+        {"name": "Mordor Orcs", "system": "Middle-Earth",
+         "group": "The Lord of the Rings™ - Evil", "faction": "Mordor",
+         "class": "Warriors", "aliases": []},
     ]
 }
 
@@ -178,6 +181,27 @@ def test_tokenless_needs_bonus():
     shutil.rmtree(root); shutil.rmtree(library)
 
 
+def test_dropped_system_mordor():
+    """Middle-earth is dropped: a 'Mordor' pack lands in _unsorted, and
+    its inner dirs cannot adopt the (filtered) Mordor Orcs entry."""
+    root = Path(tempfile.mkdtemp(prefix="sorter_test_src_"))
+    write(root / "Mordor pack" / "boy.stl")
+    library = make_lib()
+    items = sort_plan([root / "Mordor pack" / "boy.stl"], library,
+                      get_catalog(), provenance={"Mordor pack": "Mordor pack.zip"},
+                      root=root)
+    it = items[0]
+    assert "Middle-Earth" not in str(it.target), it.target
+    assert "_unsorted/Mordor pack" in str(it.target).replace("\\", "/")
+    shutil.rmtree(root); shutil.rmtree(library)
+
+
+def test_load_catalog_filters_dropped():
+    cat = load_catalog_path()
+    assert not any(e["system"] == "Middle-Earth" for e in cat)
+    assert any(e["name"] == "Warp Spiders" for e in cat)
+
+
 def test_nonprov_rehome():
     """Loose noise file inside a named dir re-homes to the dir's unit."""
     root = Path(tempfile.mkdtemp(prefix="sorter_test_src_"))
@@ -222,7 +246,8 @@ def main():
     catalog = load_catalog_path()
     for fn in (test_prov_dir_rehome, test_prov_weak_tier, test_nonprov_rehome,
                test_cross_system_needs_token_containment, test_apply_weak_doc,
-               test_weak_needs_shared_token, test_tokenless_needs_bonus):
+               test_weak_needs_shared_token, test_tokenless_needs_bonus,
+               test_dropped_system_mordor, test_load_catalog_filters_dropped):
         fn()
         print("OK", fn.__name__)