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

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1779560665,
+        "narHash": "sha256-tpyBcxPpcQb8ukyNF7DoCwfSY3VPsxHoYwj00Cayv5o=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "64c08a7ca051951c8eae34e3e3cb1e202fe36786",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}