Dartdoc Conventions — the public surface is a contract

Enforces Effective-Dart documentation on the public surface — a `///` doc on every public class/method/getter/field/typedef, a one-sentence standalone summary that says WHY plus units/ranges/nullability/throws/side-effects (never a restatement of the name), verb-phrase method docs, "Whether…" boolean getters, `[bracket]` cross-links, one `library;` doc per exported barrel, in-body `//` that explains why not what, and the enforced invariant restated at its enforcement point — backed by `public_member_api_docs` and `dangling_library_doc_comments` as analyzer errors. Use when adding or reviewing a public API, a Notifier/provider, a Service interface, a sealed Failure, a valu…