feat: add panes and shared-buffer splits

README.md

@@ -26,6 +26,8 @@ Vamp starts in Normal mode. Press `i`, type text, then Esc to return to Normal.
 | Edit completion snippet arguments | Type to replace the highlighted argument; Tab / Shift+Tab moves next/previous |
 | Save in Normal | Space then `w`, or `:w` then Enter |
 | Undo/redo in Normal | `u` / `U` (Shift+u) |
+| Split panes in Normal | `_` above/below; `\|` left/right; Option/Alt+h/j/k/l changes focus |
+| Close a pane | `:pane.close`; `:pane.force_close` explicitly discards exclusively displayed unsaved edits |
 | Delete/change whole lines in Normal | `dd` / `cc`; leading counts work (`3dd`, `2cc`) |
 | Search in Normal | `/`, type a literal query, Enter; `n`/`N` for next/previous |
 | Select/copy/cut | `v` characterwise or `V` whole rows, move to extend, then `y`/`d` in Visual |