feat: add evolution_target derivation and cell

tests/unit/test_split_selection.py

@@ -0,0 +1,12 @@
+from app.harness.split_selection import cell_of, evolution_target_of
+
+
+def test_evolution_target_mapping():
+    assert evolution_target_of("extraction_failure") == "tool"
+    assert evolution_target_of("search_failure") == "skill"
+    assert evolution_target_of("reasoning_failure") == "skill"
+    assert evolution_target_of("mixed") == "system"
+
+
+def test_cell_is_task_type_x_error_type():
+    assert cell_of("Counting Problem", "search_failure") == ("Counting Problem", "search_failure")