feat: add lora stack combiner node

tests/nodes/test_lora_stack_combiner.py

@@ -0,0 +1,51 @@
+from py.nodes.lora_stack_combiner import LoraStackCombinerLM
+
+
+def test_combine_stacks_preserves_order():
+    node = LoraStackCombinerLM()
+    stack_a = [
+        ("folder/a.safetensors", 0.7, 0.6),
+        ("folder/b.safetensors", 0.8, 0.8),
+    ]
+    stack_b = [
+        ("folder/c.safetensors", 1.0, 0.9),
+    ]
+
+    (combined_stack,) = node.combine_stacks(stack_a, stack_b)
+
+    assert combined_stack == stack_a + stack_b
+
+
+def test_combine_stacks_returns_second_when_first_empty():
+    node = LoraStackCombinerLM()
+    stack_b = [("folder/c.safetensors", 1.0, 0.9)]
+
+    (combined_stack,) = node.combine_stacks([], stack_b)
+
+    assert combined_stack == stack_b
+
+
+def test_combine_stacks_returns_first_when_second_empty():
+    node = LoraStackCombinerLM()
+    stack_a = [("folder/a.safetensors", 0.7, 0.6)]
+
+    (combined_stack,) = node.combine_stacks(stack_a, [])
+
+    assert combined_stack == stack_a
+
+
+def test_combine_stacks_returns_empty_when_both_empty():
+    node = LoraStackCombinerLM()
+
+    (combined_stack,) = node.combine_stacks([], [])
+
+    assert combined_stack == []
+
+
+def test_combine_stacks_allows_duplicate_entries():
+    node = LoraStackCombinerLM()
+    duplicate_entry = ("folder/shared.safetensors", 0.9, 0.5)
+
+    (combined_stack,) = node.combine_stacks([duplicate_entry], [duplicate_entry])
+
+    assert combined_stack == [duplicate_entry, duplicate_entry]