ComfyUI's execution engine walks backwards from output nodes. Without
OUTPUT_NODE = True, our nodes could be silently skipped when downstream
nodes are cached — even when IS_CHANGED returns a new value.
Since all three nodes have side effects (database writes, execution
tracking), they must be marked as output nodes so ComfyUI always
includes them in the execution graph.
Adds regression tests for OUTPUT_NODE and IS_CHANGED contracts.
Bumps version to 3.1.2.
