Dogfood: enforce our own architecture rules on ourselves

[LukasNiessen]

Once the files module works, add a test suite that runs the library against its own source: common depends on nothing but the standard library and the analysis toolchain, domain modules do not depend on each other, nothing depends on the public surface, no cycles.

Both shipped siblings do this and it is the best documentation the project has.

[TristanKruse]

Implemented in e72ef36. spec/architecture_spec.rb now runs ArchUnitRuby against its own source and enforces common isolation/external allowlisting, no cross-domain dependencies, no implementation dependency on the public surface, and a cycle-free library graph. Full CI is green: https://github.com/LukasNiessen/ArchUnitRuby/actions/runs/31463991701