test: add deeper integration and round-trip tests across DSLs

[rororowyourboat]

Problem

Most DSL integration tests only verify "compiles without error" + count assertions. Several gaps:

Missing test patterns

1. Round-trip tests: declare → compile → verify → serialize → deserialize → verify
2. Cross-DSL composition tests: e.g., stockflow + control in same composition
3. Parametric tests: varying composition depths (2, 5, 10 block trees), port naming patterns, type constraint variations
4. Error condition coverage: Most domain checks (SF-001..005, CS-001..006, DFD-001..005, SM-001..006) are tested for pass but not all failure modes
5. Edge cases: empty models, single-element models, degenerate canonical forms

Packages most affected

• gds-control: 101 tests, limited model variations (SISO, MIMO, open-loop only). No tests forcing CS-001/CS-004 errors.
• gds-software: Cross-diagram tests exist but only verify compilation completes. No cross-diagram wiring compatibility tests.
• gds-business: Same pattern — shallow integration tests.
• gds-stockflow: Good unit tests but integration tests use only one fixture.

Not affected

• gds-framework: 352 tests, well-covered
• gds-games: 279 tests including 67 canonical bridge tests