Roadmap: one NIF contract, and a duplicate-type diagnostic

[dannote]

Two additions from the vize_ex work:

• Before 1.0.0: duplicate @type declarations. RustQ reads @type attributes before Elixir rejects the duplicate, and Type.type_aliases/1 silently keeps the later one. It should be a RustQ.Diagnostic, with a case in the typespec matrix.
• 1.x: one contract for precompiled NIFs. Stubs with specs from defnif in build: false, load: false mode, wrapper/*_impl signature diagnostics, and public types generated from the codegen module with types_source. Types flow one way, from the codegen module outward. Reading documented types from lib/ is set aside, and the entry says why.