REPL builtins live in the REPL layer

The line-editor builtins prefixed _ed-*, and the editor types they need, live in the ral binary’s REPL layer, not in ral-core (f896dac, 2026-05-14 moved them out of core; the _ed-* family has since grown there).

The REPL is a layer above the core engine. The core descriptor describes the language and runtime; interactive editing concerns — clipboard, hyperlinks, line editing — are a frontend matter and do not belong in the engine’s builtin set. Folding them into core would entangle the language definition with one particular interactive frontend.

This keeps the boundary clean: ral-core knows nothing about the REPL, and the REPL adds its builtins on top.

See also core, the forthcoming repl, and the REPL design direction in repl-architecture.