ggrust 0.7.1

Added

• streamplot(x, y, u, v, density=...): matplotlib's streamlines computed in Rust, no matplotlib needed. Returns a StreamplotSet with lines (x, y, group) and arrows (x, y, xend, yend, group) polars frames, ready for geom_path() + geom_segment(arrow=). Lines and arrows match matplotlib 3.10 exactly on the reference cases. Masked arrays and NaN/inf are treated as missing; a density that would need more than 1e8 mask cells raises an error instead of aborting.