Assert mixed multiphase action boundaries

docs/window-animations-plan.md

@@ -170,7 +170,9 @@ Core rules:
 - Each phase is a discrete animation using the full curve.
 - A phase performs only one action kind per window: move or scale.
 - Movement and scaling are split by axis.
-- No diagonal motion.
+- No diagonal motion. Mixed-action phases may combine different per-window
+  actions, but no single window may move or resize on more than one axis in one
+  step.
 - A window or synchronized group owns its own timeline.
 - New layout changes interrupt only windows/groups with changed destinations.
 - Current hierarchy and target hierarchy both matter. The planner must know
@@ -270,6 +272,7 @@ Tests:
 - bounded generated split-tree corpus produces identical plans on repeated runs
 - unsupported and invalid candidate plans produce exact expected diagnostics
 - mixed-action phases are accepted only under exact continuous validation
+- diagonal per-window motion remains a hard rejection even inside mixed phases
 - child waits for parent/container-space phases when moving upward into a
   toplevel peer position
 - mono-mode tab switches do not animate, while entering/exiting mono can animate