Mirrors eunoia::plotting::PlacementStrategy::default(): raycast
exterior solver, POI tether, proportional margin. Per-strategy knobs
iterations (force-directed) and min_gap (elbow) live alongside in
the flat shape that the Rust FFI expects; the user-facing sublists in
eulerr_options(labels = list(force_directed = ..., elbow = ...)) are
flattened into this shape by plot.euler() before placement runs.
default_placement_opts()