Add retained spawn-out animations

docs/window-animations-plan.md

@@ -14,8 +14,9 @@ be handled deliberately.
   in-flight windows keep their existing timelines.
 - Spawn-in uses scale and position for newly mapped tiled and floating app
   windows. Layer-shell, overlay, override-redirect, and fullscreen surfaces do
-  not use this path. Spawn-out requires retained visual content after the live
-  node is gone and remains deferred.
+  not use this path. Spawn-out uses retained visual content after the live node
+  is gone, when a stable retained surface tree can be captured before unmap or
+  destroy.
 - Command-driven tile-to-float and float-to-tile transitions may animate.
   Protocol drag/drop paths do not.
 - The no-overlap multiphase system is a separate phase after the linear path is
@@ -90,7 +91,8 @@ Initial scope:
 - Live client buffers are rendered in Phase 1. Retained content freezing is
   deferred, but animated windows must still be clipped to their presentation
   bounds and must preserve the existing stretch behavior for undersized contents.
-- No spawn-out.
+- Spawn-out is retained-content-only. If the surface cannot be retained safely
+  the window snaps out instead of animating an empty frame.
 - No multiphase no-overlap planner.
 
 Tests:
@@ -116,6 +118,9 @@ Initial retained-record implementation status:
   for both tiled windows and floating child contents.
 - Tile-to-float and float-to-tile transitions retain GPU/dmabuf-backed child
   contents while the presentation geometry changes.
+- Spawn-out captures retained app-window contents before XDG/Xwayland unmap or
+  destroy, then renders a detached shrinking presentation record until the
+  animation completes.
 - Retained records hold both `GfxTexture` and `SurfaceBuffer` references so the
   existing buffer release/sync path remains authoritative.
 - Single-pixel buffers can be retained as color records.