Vow::Context
Optional per-call request context. An exported method opts in by declaring
a leading parameter typed Vow::Context (or a transport's subclass); the
macro then threads the context the transport passed to Registry#dispatch
into that argument and leaves it out of the JSON-decoded args, the manifest,
and the generated client signature. A method that declares no such parameter
is unaffected — context is purely opt-in.
This base carries nothing transport-specific. A transport subclasses it with
real accessors (headers, remote IP, …); the generic [] lets a service read
string metadata without naming a concrete transport type.