picom: switch to mainline with spring physics animations

Update picom from dccsillag fork to colonelpanic8/picom spring-physics
branch, which adds spring physics animation support to mainline picom.

Spring curve syntax: spring(stiffness, dampening, mass, clamping)
- stiffness: spring constant (higher = faster)
- dampening: resistance (higher = less oscillation)
- mass: inertia (higher = slower)
- clamping: prevent overshoot (false for bounce effects)

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

nixos/overlay.nix

@@ -168,16 +168,15 @@ in
     };
   });
 
+  # Using mainline picom with spring physics animation support
+  # Branch: spring-physics on colonelpanic8/picom
   picom = prev.picom.overrideAttrs (old: {
     src = prev.fetchFromGitHub {
       repo = "picom";
-      owner = "dccsillag";
+      owner = "colonelpanic8";
-      rev = "51b21355696add83f39ccdb8dd82ff5009ba0ae5";
+      rev = "spring-physics";
-      sha256 = "sha256-crCwRJd859DCIC0pEerpDqdX2j8ZrNAzVaSSB3mTPN8==";
+      sha256 = "sha256-sN15PffeRV6x+K2XjCxx2bIh0ujb/C1GAGTUuo2wPL4=";
     };
-    nativeBuildInputs = old.nativeBuildInputs ++ [final.pcre final.gnugrep.pcre2 final.asciidoc];
-    buildInputs = old.buildInputs ++ [final.pcre];
-    nativeInstallCheckInputs = [];
     doCheck = false;
     dontCheck = true;
   });

nixos/xmonad.nix

@@ -98,25 +98,73 @@ makeEnable config "myModules.xmonad" true  {
       backend = "glx";
 
       settings = {
-        animations = true;
+        # Spring physics animations (mainline picom with spring-physics branch)
-        animation-window-mass = 1;
+        # Syntax: spring(stiffness, dampening, mass) or spring(stiffness, dampening, mass, clamping)
-        animation-dampening = 20;
+        # Set clamping to false for bounce/overshoot effects
-        animation-stiffness = 250;
+        animations = [
-        animation-clamping = false;
+          # Window move/resize animation with spring physics
-        animation-for-open-window = "zoom";
+          {
-        animation-for-unmap-window = "zoom";
+            triggers = ["geometry"];
-        animation-for-transient-window = "slide-down";
+            offset-x = {
+              curve = "spring(250, 20, 1, false)";
+              start = "window-x-before - window-x";
+              end = 0;
+            };
+            offset-y = {
+              curve = "spring(250, 20, 1, false)";
+              start = "window-y-before - window-y";
+              end = 0;
+            };
+          }
+          # Opacity fade animation
+          {
+            triggers = ["open" "show"];
+            opacity = {
+              curve = "cubic-bezier(0.25, 0.1, 0.25, 1)";
+              duration = 0.2;
+              start = 0;
+              end = "window-raw-opacity";
+            };
+            scale-x = {
+              curve = "cubic-bezier(0.25, 0.1, 0.25, 1)";
+              duration = 0.2;
+              start = 0.9;
+              end = 1;
+            };
+            scale-y = {
+              curve = "cubic-bezier(0.25, 0.1, 0.25, 1)";
+              duration = 0.2;
+              start = 0.9;
+              end = 1;
+            };
+          }
+          {
+            triggers = ["close" "hide"];
+            opacity = {
+              curve = "cubic-bezier(0.25, 0.1, 0.25, 1)";
+              duration = 0.2;
+              start = "window-raw-opacity";
+              end = 0;
+            };
+            scale-x = {
+              curve = "cubic-bezier(0.25, 0.1, 0.25, 1)";
+              duration = 0.2;
+              start = 1;
+              end = 0.9;
+            };
+            scale-y = {
+              curve = "cubic-bezier(0.25, 0.1, 0.25, 1)";
+              duration = 0.2;
+              start = 1;
+              end = 0.9;
+            };
+          }
+        ];
+
         fade-in-step = 0.028;
         fade-out-step = 0.028;
         fading = true;
-      };
 
-      wintypes = {
-        dock = {animation = "slide-down";};
-        toolbar = {animation = "slide-down";};
-      };
-
-      settings = {
         focus-exclude = ["class_g ?= 'rofi'" "class_g ?= 'Steam'"];
         rounded-corners-exclude = [
           "! name~=''" # Qtile == empty wm_class..