feat: add Nix flake for dev environment

Provide a reproducible dev shell with JDK 17 and Maven 3.9.x.
JDK 17 (LTS) is used since JDK 16 (project target) is non-LTS
and unavailable in nixpkgs; it is fully backward-compatible with
--source 16 --target 16. Maven is overridden via overlay to use
the same JDK.

flake.nix

@@ -0,0 +1,48 @@
+{
+  description = "new-shapes - Java Swing shape editor";
+
+  inputs.nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+
+  outputs = { self, nixpkgs }:
+    let
+      # JDK 16 (project target) is non-LTS and unavailable in nixpkgs.
+      # JDK 17 (LTS) is fully backward-compatible with --source 16 --target 16.
+      javaVersion = 17;
+
+      supportedSystems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
+
+      forEachSystem = f:
+        nixpkgs.lib.genAttrs supportedSystems (system: f {
+          pkgs = import nixpkgs {
+            inherit system;
+            overlays = [ self.overlays.default ];
+          };
+        });
+    in
+    {
+      overlays.default = final: prev:
+        let
+          jdk = prev."jdk${toString javaVersion}";
+        in
+        {
+          inherit jdk;
+          # Override Maven to use the same JDK version
+          maven = prev.maven.override { jdk_headless = jdk; };
+        };
+
+      devShells = forEachSystem ({ pkgs }: {
+        default = pkgs.mkShell {
+          packages = with pkgs; [
+            jdk
+            maven
+          ];
+
+          shellHook = ''
+            echo "new-shapes dev shell"
+            echo "  Java: $(java -version 2>&1 | head -1)"
+            echo "  Maven: $(mvn -version 2>&1 | head -1)"
+          '';
+        };
+      });
+    };
+}