M1: keyboard transforms: arrows reorder auto layout children, Cmd+arrows resize (#6)

- Arrow keys on children of an auto layout frame move them one place along the flow (Left/Up
  earlier, Right/Down later; grids step by a row vertically); selected neighbours move as a
  block and stop at the ends. Free layers still nudge.
- Cmd/Ctrl+arrows resize the selection by the nudge amount (Shift: big nudge), top-left fixed;
  only the changed axis becomes Fixed. New command `transform.nudgeSize {dw, dh, big?}`.
- Control channel `ui.key` arrows with `cmd` resize too.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/control-protocol.md

@@ -76,7 +76,7 @@ A mouse gesture with the current tool. `kind`: `down`, `move`, `up`, `click`, `d
 ### `ui.key {key, shift?, alt?, cmd?, ctrl?}`
 
 A key as if typed on the canvas (in vector edit mode the edit keys come first: `V` `P` `Q` `X` `Shift+B` tools, arrows nudge points, `Delete`/`Shift+Delete`, `Cmd+A`, `Cmd+J`): a menu shortcut (`{"key":"G","cmd":true}` groups), a tool key,
-`Escape`, `Delete`/`Backspace`, `Enter`, arrows (nudge; shift = big nudge). While text is being
+`Escape`, `Delete`/`Backspace`, `Enter`, arrows (nudge; shift = big nudge; auto layout children move one place along the flow; with `cmd` they resize the selection). While text is being
 edited, keys go to the text editor. Returns `{"command": id}` for shortcuts.
 
 ### `ui.type {text}`