build(ground-suite): add a separate pinned Flutter shell

Provide a separate Flutter development shell from the existing locked
Nix input and document the application build commands.

Validation: locked environment/version probe; no native application
qualification is claimed.

flake.nix

@@ -33,6 +33,23 @@
               pkgs.python3
             ];
           };
+          ground-suite = pkgs.mkShell {
+            packages = [
+              (pkgs.flutterPackages.v3_41.override {
+                supportedTargetFlutterPlatforms = [
+                  "universal"
+                  "web"
+                  "linux"
+                ];
+              })
+              pkgs.clang
+              pkgs.cmake
+              pkgs.ninja
+              pkgs.pkg-config
+              pkgs.gtk3
+            ];
+            CMAKE_BUILD_PARALLEL_LEVEL = "2";
+          };
         }
       );
     };