Extracts the split-pane rules from the legacy shell's window_context/split.rs into a zero-dependency workspace crate: the layout tree (split/remove/collapse, path addressing), the geometry pass (floor + per-side cell clamping, divider rects with tree paths), divider drag rules (unclamped pointer ratio, preview pinning in the close zone, drag-to-close margins, whole-cell commit quantization) and directional focus (4x perpendicular-drift penalty). Math ported verbatim and locked by 12 unit tests. The legacy implementation stays frozen in-tree per the P4 ruling; the GPUI layer inside nebula_app is the intended consumer.