Tests verify:
- Legacy mode: vars merged globally (A sees B's VAR, can access UNIQUE_B)
- Scoped mode: vars isolated (A sees own VAR, cannot access UNIQUE_B)
- Inheritance: includes can still access root vars (ROOT_VAR)
Test structure:
- testdata/scoped_includes/ with main Taskfile and two includes
- inc_a and inc_b both define VAR with different values
- Cross-include test shows A trying to access B's UNIQUE_B
