overlays: consolidate Abseil BMI2 workaround under patches

Extract the header fix into a standalone patch and colocate the
Electron, Deno, and Signal overrides in patches/abseil. Preserve the
x86-64-v3 restriction and document the accepted upstream fix while
waiting for bundled dependency updates.

Import the Abseil overrides through the patches overlay and register
the torchcodec override. Leave PostgreSQL output checks in
x86-64-v3-workarounds.nix.

overlays/patches/default.nix

@@ -1,9 +1,12 @@
-_final: prev: {
+_final: prev:
+(import ./abseil { inherit prev; })
+// {
   gnutls = import ./gnutls { inherit (prev) gnutls; };
 
   pythonPackagesExtensions = prev.pythonPackagesExtensions ++ [
     (_pythonFinal: pythonPrev: {
       scipy = import ./scipy { inherit (pythonPrev) scipy; };
+      torchcodec = import ./torchcodec { inherit (pythonPrev) torchcodec; };
     })
   ];
 }