picom: add scale animation for size changes in geometry trigger

When both size AND position change (common in tiling WMs), picom's
win_position_changed() returns false. Only size_changed is true.
This means the geometry trigger fires but only scale-x/y change,
not offset-x/y.

Added scale-x and scale-y with spring physics to the geometry
animation so windows animate properly when resized.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nixos/xmonad.nix

@@ -127,6 +127,8 @@ makeEnable config "myModules.xmonad" true  {
         # Set clamping to false for bounce/overshoot effects
         animations = (
           # Window move/resize animation with spring physics
+          # Note: "geometry" is alias for "size" + "position" triggers
+          # position_changed is FALSE if size also changed, so we need scale-x/y too
           {
             triggers = ["geometry"];
             offset-x = {
@@ -139,6 +141,16 @@ makeEnable config "myModules.xmonad" true  {
               start = "window-y-before - window-y";
               end = 0;
             };
+            scale-x = {
+              curve = "spring(250, 20, 1, false)";
+              start = "window-width-before / window-width";
+              end = 1;
+            };
+            scale-y = {
+              curve = "spring(250, 20, 1, false)";
+              start = "window-height-before / window-height";
+              end = 1;
+            };
           },
           # Opacity fade animation
           {