Test coverage gaps from the L3 review #25

Is your feature request related to a problem? Please describe.
Some test coverage gaps noted in the reviews of PR #16, none hiding a known bug:

1. TestGateL2_SupersetOfL1 builds its L2 input without external_symbols, so the superset check skips the library-edge path.
2. Satellite call nodes' parent assignment is untested for calls in folded for init/post, case lists, select comm clauses and range expressions. L4 port wiring will depend on it.
3. No exact-edge CFG fixture covers range over an integer or over an iterator function (Go 1.22+), because testdata/dataflow declares go 1.21.

Describe the solution you'd like

1. Add external_symbols to the superset gate.
2. Add parent assertions for each listed construct.
3. Add a Go 1.23+ fixture module with exact-edge tests for both range forms.

Describe alternatives you've considered
None.

Additional context
Do item 2 before L4.