Add Nix flake for reproducible builds

.gitignore

@@ -1,3 +1,6 @@
+# Nix build output symlink
+/result
+
 /.metadata/
 /workspace/
 cachedir/

flake.nix

@@ -0,0 +1,53 @@
+# Modelio – open-source UML/BPMN modeling tool
+#
+# Usage:
+#   nix build          Build Modelio
+#   nix run            Build and launch Modelio
+#   nix develop        Enter dev shell (Maven + JDK 11 + native libs)
+#   nix build .#container        Build OCI container image
+#   nix run .#container-run      Load and run container with Docker
+#   nix flake check              Run smoke tests
+#
+# See ./nix/README.md for full documentation.
+{
+  description = "Modelio – open-source UML/BPMN modeling tool";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
+    flake-utils.url = "github:numtide/flake-utils";
+
+    # Pinned Modelio source for maven dependency fetching.
+    # This ensures the maven deps FOD only changes when you explicitly
+    # run: nix flake update modelio-src
+    modelio-src = {
+      url =
+        "github:randomizedcoder/Modelio/53eb60381eac57b313ea862208b3d38652bf59b9";
+      flake = false;
+    };
+  };
+
+  outputs = { nixpkgs, flake-utils, modelio-src, ... }:
+    flake-utils.lib.eachSystem [ "x86_64-linux" ] (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+        mvnDeps =
+          pkgs.callPackage ./nix/maven-deps.nix { inherit modelio-src; };
+        modelio =
+          pkgs.callPackage ./nix/package.nix { inherit modelio-src mvnDeps; };
+        container = pkgs.callPackage ./nix/container.nix { inherit modelio; };
+      in {
+        packages = {
+          default = modelio;
+          inherit modelio;
+          inherit container;
+          container-run =
+            pkgs.callPackage ./nix/container-run.nix { inherit container; };
+        };
+
+        devShells.default = pkgs.callPackage ./nix/shell.nix { };
+
+        checks.default = pkgs.callPackage ./nix/test.nix { inherit modelio; };
+
+        formatter = pkgs.nixfmt-classic;
+      });
+}