✅ Fuse demons does not mutate input

Category	Ability
Status	Passing
Test	tests/test_fusion.py::test_fuse_demons_does_not_mutate_input

Verify copy-on-write — fuse_demons must not mutate the input state.

Preconditions

• P1 owns Murmur (#002) and Duban (#001) in lane 0

Action

• fuse_demons(state, murmur, duban)

Expected Postconditions

• Original state.demons still has 2 demons

• Original murmur.is_fused is still False

• Original murmur.state is still READIED

Assertions

assert len(state.demons) == original_demon_count
    assert state.demons[0].is_fused == original_top_is_fused