HTTP effect facade + backend behaviour.
A node's effect-shell calls Bloccs.Effects.HTTP.post/3 / get/2 against the
capability struct in ctx.effects.http. The facade dispatches to whichever
backend built that struct — Bloccs.Effects.HTTP.Mock in tests,
Bloccs.Effects.HTTP.Req in production — so node source never names a
concrete backend. Backends @behaviour Bloccs.Effects.HTTP.
Allowlist/method enforcement is the backend's responsibility (every shipped backend enforces it before doing I/O); the facade is pure dispatch.
Summary
Types
A backend capability struct (e.g. %HTTP.Mock{} / %HTTP.Req{}).
A [effects].http declaration: allowed hosts + methods.