feat(backend): migrate production backend to Rust

Replace the Go backend with the Rust workspace and route Android/iOS through
UniFFI bindings. Include the migrated extension runtime, network, providers,
media metadata, downloads and library operations, with version 5.0.0+147.

Remove Go sources, adapters, native build selection and CI dependencies.
Build Rust unconditionally and lock the iOS Rust pod using a relative path.
Retain legacy source and migration evidence in a local ignored archive.

Validation: Android Kotlin compile and 53 native tests; Swift Rust-branch
equivalence and syntax; CocoaPods install; workflow, shell, Ruby, plist and
diff checks. Reuse the preceding 100 Rust tests, fmt/Clippy and five-ABI
build checkpoint; no new APK or iOS application build for this cleanup.

Known follow-up: URL/URLSearchParams globals are missing from the Rust JS
runtime. A controlled extension replay confirms a URL-resolution regression;
this commit does not fix that runtime gap.

test/platform_bridge_post_processing_test.dart

@@ -0,0 +1,93 @@
+import 'dart:convert';
+
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spotiflac_android/services/platform_bridge.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  const channel = MethodChannel('com.zarz.spotiflac/backend');
+
+  tearDown(() {
+    TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+        .setMockMethodCallHandler(channel, null);
+  });
+
+  test(
+    'native replacement is exposed to the download queue as file_path',
+    () async {
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+          .setMockMethodCallHandler(channel, (call) async {
+            expect(call.method, 'runPostProcessingV2');
+            expect(call.arguments, {
+              'input': jsonEncode({
+                'item_id': 'queue-item',
+                'path': '/music/original.flac',
+              }),
+              'metadata': jsonEncode({'title': 'Example'}),
+            });
+            return jsonEncode({
+              'success': true,
+              'new_file_path': '/music/converted.m4a',
+            });
+          });
+      final result = await PlatformBridge.runPostProcessingV2(
+        '/music/original.flac',
+        metadata: {'title': 'Example'},
+        itemId: 'queue-item',
+      );
+      expect(result['file_path'], '/music/converted.m4a');
+      expect(result['new_file_path'], '/music/converted.m4a');
+    },
+  );
+
+  test(
+    'SAF keeps the destination URI instead of exposing a staging path',
+    () async {
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+          .setMockMethodCallHandler(channel, (call) async {
+            expect(call.arguments, {
+              'input': jsonEncode({'uri': 'content://example/document/track'}),
+              'metadata': '',
+            });
+            return jsonEncode({
+              'success': true,
+              'new_file_path': '/cache/processed.flac',
+              'new_file_uri': 'content://example/document/track',
+            });
+          });
+      final result = await PlatformBridge.runPostProcessingV2(
+        'content://example/document/track',
+      );
+      expect(result['file_path'], 'content://example/document/track');
+    },
+  );
+
+  test(
+    'legacy results, no-op hooks and failures retain their contracts',
+    () async {
+      for (final response in <Map<String, dynamic>>[
+        {'success': true, 'file_path': '/music/legacy.flac'},
+        {'success': true, 'new_file_path': ''},
+        {'success': false, 'error': 'processing failed'},
+      ]) {
+        TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+            .setMockMethodCallHandler(
+              channel,
+              (_) async => jsonEncode(response),
+            );
+        final result = await PlatformBridge.runPostProcessingV2(
+          '/music/original.flac',
+        );
+        if (response['success'] == true) {
+          expect(
+            result['file_path'],
+            response['file_path'] ?? '/music/original.flac',
+          );
+        } else {
+          expect(result, response);
+        }
+      }
+    },
+  );
+}