Cartograph: inspect Swift dependencies and explain reachability

Running Cartograph on its own codebase turned up two type-level dependency cycles while its module-level check was passing. Swift already rejects circular module imports, so that check couldn't tell me about dependencies between types within a module. The project's CI now checks for type-level cycles too. Cartograph is an MIT-licensed CLI that reads the compiler's index store. It builds a graph you can query for a declaration's users, dependencies, and reachability, including why it is retained...