Python/OpenCV proof of concept plus first native Swift port

Snapshot before the Python POC is removed. The POC (main.py, effects.py,
panel.py, shaders.py, effects/*.glsl, flake.nix) is the reference for the
native app under native/.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

flake.nix

@@ -0,0 +1,53 @@
+{
+  description = "motionlab - realtime camera + motion graphics playground";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+
+  outputs = { self, nixpkgs }:
+    let
+      systems = [ "aarch64-darwin" "x86_64-darwin" "x86_64-linux" "aarch64-linux" ];
+      forAllSystems = f: nixpkgs.lib.genAttrs systems (system: f nixpkgs.legacyPackages.${system});
+
+      # not in nixpkgs; sends frames to the OBS virtual camera (macOS) or v4l2loopback (Linux)
+      pyvirtualcamFor = ps: ps.buildPythonPackage rec {
+        pname = "pyvirtualcam";
+        version = "0.15.0";
+        pyproject = true;
+        src = nixpkgs.legacyPackages.${ps.python.stdenv.hostPlatform.system}.fetchFromGitHub {
+          owner = "letmaik";
+          repo = "pyvirtualcam";
+          tag = "v${version}";
+          fetchSubmodules = true;
+          hash = "sha256-JtFchhuvVYYhrYOOcsHPC79/Iezyo5QppV4tY9nrjgQ=";
+        };
+        build-system = [ ps.setuptools ps.pybind11 ];
+        dependencies = [ ps.numpy ];
+        doCheck = false;
+        pythonImportsCheck = [ "pyvirtualcam" ];
+      };
+
+      pythonFor = pkgs: pkgs.python3.withPackages (ps: [
+        ps.numpy
+        ps.moderngl
+        (pyvirtualcamFor ps)
+        # macOS gets a Cocoa window out of the box; Linux needs GTK for cv2.imshow
+        (if pkgs.stdenv.hostPlatform.isLinux
+         then ps.opencv4.override { enableGtk3 = true; }
+         else ps.opencv4)
+      ]);
+    in
+    {
+      devShells = forAllSystems (pkgs: {
+        default = pkgs.mkShell { packages = [ (pythonFor pkgs) ]; };
+      });
+
+      apps = forAllSystems (pkgs: {
+        default = {
+          type = "app";
+          program = toString (pkgs.writeShellScript "motionlab" ''
+            exec ${pythonFor pkgs}/bin/python ${self}/main.py "$@"
+          '');
+        };
+      });
+    };
+}