Add Nix flake dev shell for building on NixOS

Provides JDK 21 and an Android SDK (platforms 36/36.1, build-tools
36.0.0) via `nix develop`. Points AGP at the SDK's aapt2 and Gradle at
the Nix JDK, since the binaries they would otherwise download do not
run on NixOS.

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

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1791956154,
+        "narHash": "sha256-jBYGPi4udQ9uo4KQp9m2J0546nJVQitG15gvZSaVVAs=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "55bfb4d7437bd4e09ab7cba58a43bd68eb310044",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,64 @@
+{
+  description = "FatihsMG4 — Android Automotive build environment";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs }:
+    let
+      systems = [ "x86_64-linux" "aarch64-linux" ];
+      forAllSystems = f: nixpkgs.lib.genAttrs systems (system: f (import nixpkgs {
+        inherit system;
+        config = {
+          allowUnfree = true;
+          android_sdk.accept_license = true;
+        };
+      }));
+
+      # Must match app/build.gradle (compileSdk 36.1) and EVHardware/lib (compileSdk 36).
+      buildToolsVersion = "36.0.0";
+    in
+    {
+      devShells = forAllSystems (pkgs:
+        let
+          android = pkgs.androidenv.composeAndroidPackages {
+            platformVersions = [ "36" "36.1" ];
+            buildToolsVersions = [ buildToolsVersion ];
+            includeEmulator = false;
+            includeNDK = false;
+            includeSources = false;
+            includeSystemImages = false;
+          };
+          sdk = "${android.androidsdk}/libexec/android-sdk";
+          # gradle/gradle-daemon-jvm.properties pins the daemon to JDK 21.
+          jdk = pkgs.jdk21;
+        in
+        {
+          default = pkgs.mkShell {
+            packages = [ jdk android.androidsdk ];
+
+            ANDROID_HOME = sdk;
+            ANDROID_SDK_ROOT = sdk;
+            JAVA_HOME = jdk.home;
+
+            # AGP otherwise downloads an aapt2 binary from Maven, which is dynamically linked
+            # against a generic FHS and does not run on NixOS. Use the SDK's patched one.
+            # Gradle is also pointed at the Nix JDK so the daemon toolchain criteria are met
+            # locally instead of downloading a (non-NixOS) JDK through foojay.
+            GRADLE_OPTS = builtins.concatStringsSep " " [
+              "-Dorg.gradle.project.android.aapt2FromMavenOverride=${sdk}/build-tools/${buildToolsVersion}/aapt2"
+              "-Dorg.gradle.java.installations.paths=${jdk.home}"
+              "-Dorg.gradle.java.installations.auto-download=false"
+            ];
+
+            shellHook = ''
+              echo "sdk.dir=$ANDROID_HOME" > local.properties
+              echo "FatihsMG4 dev shell — JDK $(java -version 2>&1 | head -1 | cut -d'"' -f2), SDK at $ANDROID_HOME"
+              echo "  ./gradlew assembleStableDebug    build a debug APK"
+              echo "  ./gradlew testStableDebugUnitTest  run unit tests"
+            '';
+          };
+        });
+    };
+}