Realises the SPEC §7 / §12.3 layering commitment at the registry
boundary: the read accessors only ever invoke read and
subscribe, and the parameter type proves it. A caller that hands
the registry a method reference cannot then reach for commit,
input, derived, hydrate, exportModel, … through the same
value — the surface is sealed at the type system rather than at the
call site.
Graph is assignable to this slice, so existing call sites that
pass a full engine handle keep compiling. The narrowing is strict —
ConflictRegistryReadGraph cannot widen back to Graph without an
explicit cast, which is the lock the
*.narrowCapability.test.ts suite asserts.
Read-side capability slice handed to ConflictRegistry.read and ConflictRegistry.subscribe.