feat(normalizer): add generation parser to persons_tree

tools/import-normalizer/tests/test_persons_tree.py

@@ -59,3 +59,27 @@ def test_parse_year_bare_out_of_range_year_is_none():
     # parse_date("1023") parses it as year 1023 (out of 1700-2100 guard).
     # The serial branch must NOT re-interpret it as a serial.
     assert persons_tree._parse_year("1023") is None
+
+
+def test_parse_generation_space():
+    assert persons_tree._parse_generation("G 3") == 3
+
+
+def test_parse_generation_no_space():
+    assert persons_tree._parse_generation("G3") == 3
+
+
+def test_parse_generation_extra_spaces():
+    assert persons_tree._parse_generation("G  0") == 0
+
+
+def test_parse_generation_trailing_garbage():
+    assert persons_tree._parse_generation("G 2         de Gruyter") == 2
+
+
+def test_parse_generation_empty():
+    assert persons_tree._parse_generation("") is None
+
+
+def test_parse_generation_none():
+    assert persons_tree._parse_generation(None) is None