Add multiphase no-overlap planner groundwork

docs/window-animations-plan.md

@@ -30,6 +30,15 @@ be handled deliberately.
   partial inspiration for tiling style and titlebar/grouping behavior.
 - Mono mode should mostly avoid animations. Exceptions are windows entering or
   exiting mono mode, where a visual transition can clarify the hierarchy change.
+- Multiphase shrink steps should not normally need to reduce a tiled window far
+  below roughly one quarter of the relevant full size. The implementation may
+  enforce a conservative sanity minimum, and pathological cases may fall back.
+- If the no-overlap planner cannot produce a legal sequence, only the affected
+  group should fall back to linear animation. This is expected to be rare for
+  valid tiling layouts.
+- When entering mono mode, the active child should animate to the mono geometry.
+  Inactive siblings may snap invisible. Floats may overlap normally and do not
+  need the no-overlap planner.
 
 ## Texture Freezing Decision
 
@@ -188,12 +197,22 @@ Preferred approach:
   animated visual actors.
 - Derive every leaf's per-phase rect from one phase schedule so parent and child
   effects cannot compose into forbidden motion.
+- Build the planner as pure geometry first. Live integration should collect
+  eligible leaf `(old, new)` rects across a command-driven layout pass, then
+  submit planner-produced phases as a batch. Per-node `tl_change_extents` calls
+  are too incremental to plan safely by themselves.
 - Add container-level grouping only after the leaf planner proves correct.
 - Include hierarchy-transition metadata in the planner input: source parent,
   target parent, source depth, target depth, and whether the window is ascending,
   descending, or staying at the same hierarchy level.
 - For mono containers, suppress ordinary in-mono focus/tab changes. Animate only
   transitions into mono, out of mono, or across the mono boundary.
+- When entering mono, the active child animates to the full mono area and
+  inactive siblings snap invisible. When exiting mono, ordinary tiled geometry
+  may animate from the mono child where that produces a clear hierarchy
+  transition.
+- If a legal no-overlap sequence cannot be found for a group, fall back to the
+  linear animator for that group only. Float windows are outside this invariant.
 
 Tests: