docs/emitters: detect client-level parameters by structure #719

Problem

Parameters that every operation in a group or service takes (an account or tenant ID in the path, an API version header) are usually exposed once, on the client, rather than on every method. Nothing in the design says how they are found, and the easy rule, matching by parameter name, promotes unrelated parameters that happen to share a name.

Proposal

A plan fact, marked Inferred and overridable by hints: a parameter is client-level for a scope when every operation in the scope uses the same component parameter (Parameter.Provenance names the component). Never by name alone.