Add Nix flake: devShell + hermetic nonRoot debug APK build

- flake.nix: devShells.default (JDK 17 + Android SDK 34 / NDK 27.0.12077973 + Gradle)
  and packages.moonlight-android (assembleNonRootDebug via gradle mitmCache)
- deps.json: offline Maven lockfile (195 artifacts)
- nix-deps-fixup.gradle: skip androidTest variant ambiguity during dep capture
- README: Building with Nix section

nix-deps-fixup.gradle

@@ -0,0 +1,21 @@
+// Workaround for the nixpkgs Gradle mitm-cache hook.
+//
+// The hook's `nixDownloadDeps` task force-resolves every resolvable configuration
+// to record all dependencies. AGP's androidTest/test classpaths reference the app
+// project itself and cannot disambiguate its published variants, so resolving them
+// throws a "cannot choose between the following variants of project :app" error and
+// aborts dependency capture.
+//
+// The hermetic build target is `assembleNonRootDebug`, which never compiles tests,
+// so we mark every test configuration as non-resolvable. `nixDownloadDeps` filters
+// on `canBeResolved`, so these are simply skipped, and the real build does not touch
+// them either.
+gradle.projectsLoaded {
+  rootProject.allprojects { proj ->
+    proj.afterEvaluate {
+      proj.configurations
+        .matching { it.name.toLowerCase().contains("test") }
+        .all { it.setCanBeResolved(false) }
+    }
+  }
+}