Add goldengen accounting assertion, worked example, and testing docs

[sourcehawk]

Towards #134.

Completes the goldengen feature with its completeness check, a runnable example, and the testing documentation.

What this adds

• AssertComplete accounting on *Generator[T]. Called from a consumer's TestMain as os.Exit(gen.AssertComplete(m.Run())), it proves the universe of registered mutation names across all fixtures equals union(Requires names) ∪ Exclude. It returns the incoming code unchanged when the tests already failed or accounting holds; otherwise it prints the violations (unaccounted mutation, stale Exclude/Requires name, empty name) to stderr and returns nonzero. Registration is version-independent, so each fixture is built once to gather the registered set.
• Worked example under examples/version-matrix/: a StatefulSet built through goldengen with three version-gated mutations over the shared ExampleApp CRD. It exercises Run (with a -update flag wired through WithUpdate) and AssertComplete from TestMain. The committed testdata/ shows the two generated regime goldens and the coverage manifest for the version universe 8.7.0, 8.8.2, 8.9.0.
• docs/testing.md: the golden helpers (AssertYAML/AssertComponentYAML/Serialize/SerializeComponent) and the full goldengen workflow (matrix declaration, the four Requires/Forbids assertions, firing-set classification with a diagram, the ascending version-ordering convention, the manifest, and the LoadMatrix YAML loader). Linked from the README documentation table.

Verification

make all and make build-examples both pass.

Note: the LoadMatrix section documents the loader that ships in the sibling PR (#135) against its locked signature, so the feature's documentation is complete in one place.

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