fix(aaron): workaround compiler-rt 18 darwin build via overlay

modules/hosts/aaron/configuration.nix

@@ -61,13 +61,33 @@ in
   networking.hostName = "aaron";
   networking.computerName = "aaron";
 
-  # Packages pinned to older nixpkgs until binary cache is available.
-  # Remove entries once the package has a cached build for aarch64-darwin.
-  # nixpkgs.overlays = [
-  # (_final: _prev: {
-  # inherit (nixpkgs-darwin-pins.legacyPackages.aarch64-darwin) deno;
-  # })
-  # ];
+  # Workaround for NixOS/nixpkgs#480849: building llvmPackages_18.compiler-rt
+  # on Darwin fails because Apple SDK 26.4 ships libc++ 21, which dropped the
+  # fallbacks for __builtin_ctzg/__builtin_clzg — builtins that Clang 18 does
+  # not recognize. Mirror the fix from NixOS/nixpkgs#523142 by disabling the
+  # C++ components of compiler-rt 18 here.
+  #
+  # TODO: remove this overlay once nixos-unstable advances past the fix.
+  # Trigger: NixOS/nixpkgs#523142 (or its replacement) is merged AND lands in
+  # nixos-unstable. Verify by deleting the overlay block below and running
+  # `nix build .#darwinConfigurations.aaron.system` — if it succeeds, delete
+  # for good. If it still errors with `__builtin_ctzg`, keep the overlay.
+  nixpkgs.overlays = [
+    (_final: prev: {
+      llvmPackages_18 = prev.llvmPackages_18.overrideScope (
+        _llvmFinal: llvmPrev: {
+          compiler-rt-libc = llvmPrev.compiler-rt-libc.overrideAttrs (old: {
+            cmakeFlags = (old.cmakeFlags or [ ]) ++ [
+              (lib.cmakeBool "COMPILER_RT_BUILD_XRAY" false)
+              (lib.cmakeBool "COMPILER_RT_BUILD_LIBFUZZER" false)
+              (lib.cmakeBool "COMPILER_RT_BUILD_MEMPROF" false)
+              (lib.cmakeBool "COMPILER_RT_BUILD_ORC" false)
+            ];
+          });
+        }
+      );
+    })
+  ];
 
   system = {
     primaryUser = "soft";