Move HeuristicScores type and ScoreHeuristic logic into pkg/heuristic
with zero external deps (stdlib only). pkg/lem delegates via type alias
and wrapper function — fully backward compatible. Enables EaaS to
cross-compile for Linux without dragging in go-ml/go-mlx/go-duckdb.
Also adds missing //go:build tag to backend_mlxlm.go.
Co-Authored-By: Virgil <virgil@lethean.io>
