Add initial core helper smoke tests

[bernalde]

Summary

• Add fast unit coverage for topological_bfs ordering and cycle detection.
• Add a state-vector split test for NameAnalysis.get_dict_states.
• Add a local Newton interpolation check against a hand-computed quadratic value.

Traceability

Refs #7.
Depends on #94 / #6 and is stacked on ci/assimulo-integration.
Refs #1, #2, CryPTSys#106, and CryPTSys#107.

Verification

• pytest --collect-only
• pytest tests/ -m "not assimulo"
• pytest tests/ -m assimulo (local environment without Assimulo: 6 selected, 6 skipped)

Actions

GitHub Actions should run the core job and the informational Assimulo job inherited from #94.