rfc: boundary-edge taxonomy (http/rpc/queue) as canonical edges

[rahlk]

RFC: boundary-edge taxonomy as canonical Edge kinds — http, rpc, queue (later ffi).

Must cover

• Edge shape: consumer callsite URI → provider entry-point URI, the matched route/topic, and the evidence that produced the match.
• All boundary edges are structural confidence by construction; the RFC defines how match quality is expressed on top of that.
• The division of labor: what backends must emit (routes, client calls) vs what the SDK links.

Part of #196. Branch: chore/issue-<n>.