Enables the `steel` Cargo feature on the burin fork (which now builds after fixing two API-drift compile errors -- BufferLine rename, DiffProvider mutability -- against a rebased helix-editor/helix main) and wires the badjuju Steel plugin cogs directly from its flake input source tree. helix.scm requires badjuju.scm by absolute store path rather than copying into xdg.configFile, since badjuju.scm requires its sibling badjuju-core.scm by a path relative to its own location -- a copy would orphan that relative require (confirmed against a real build: the failure only ever surfaces as a silent TUI popup, never logged).