refactor(native): share errorJson and retire Go-era names in native comments

MainActivitySafIo and SafDownloadHandler each defined an identical
errorJson. Keep one internal top-level function in MainActivitySafIo
and drop the private copy. errorJson, isMediaStoreUri, extFromFileName
and extFromMimeType never used their MainActivity receiver, so they
become plain top-level functions; call sites are unchanged.

Rust is the only backend. Rename the iOS invokeGoMethod, which
dispatches the OS-owned channel methods, to invokeOsMethod, and reword
the comments that still described Go calls, the Go progress stream, the
Go metadata reader or a "native migration boundary" so they describe
the current backend. Channel method names and reply keys are unchanged.

android/app/src/main/kotlin/com/zarz/spotiflac/CoreBackend.kt

@@ -160,7 +160,7 @@ internal fun withCoreFFmpegExecution(
     }
 }
 
-/** Native migration boundary; each process selects one stateful backend. */
+/** Native boundary to the Rust backend; each process holds one stateful instance. */
 internal interface CoreBackend {
     val implementation: String
     fun invokeApplication(method: String, arguments: Any?): Any?

android/app/src/main/kotlin/com/zarz/spotiflac/DownloadService.kt

@@ -362,7 +362,7 @@ class DownloadService : Service() {
     internal val nativeWorkerTerminalStatuses = mutableMapOf<String, String>()
     internal val nativeReplayGainEntries = mutableListOf<JSONObject>()
     internal val nativeReplayGainRequestAlbumKeys = mutableMapOf<String, String>()
-    // One coordinator polls Go's delta progress stream for all native workers.
+    // One coordinator polls the backend's delta progress stream for all native workers.
     // Individual workers consume this cache instead of parsing the complete
     // multi-progress JSON independently.
     internal val nativeWorkerProgressLock = Any()

android/app/src/main/kotlin/com/zarz/spotiflac/DownloadServiceSnapshot.kt

@@ -14,7 +14,7 @@ import java.util.concurrent.atomic.AtomicReference
 // Native-worker item state snapshots for the Flutter side.
 
 /**
- * The compact subset of Go progress needed by the Android worker UI. Keeping
+ * The compact subset of backend progress needed by the Android worker UI. Keeping
  * this as a Kotlin value avoids sharing mutable JSONObject instances between
  * the polling coroutine and worker coroutines.
  */

android/app/src/main/kotlin/com/zarz/spotiflac/MainActivitySafIo.kt

@@ -17,7 +17,7 @@ import java.util.Locale
 // SAF/MediaStore URI IO helpers: temp copies, writes, sidecars, and the
 // SAF post-processing pipeline.
 
-internal fun MainActivity.errorJson(message: String): String {
+internal fun errorJson(message: String): String {
         val obj = JSONObject()
         obj.put("success", false)
         obj.put("error", message)
@@ -31,7 +31,7 @@ internal fun MainActivity.errorJson(message: String): String {
      * which require READ_MEDIA_AUDIO / READ_EXTERNAL_STORAGE permission
      * instead of SAF tree permission.
      */
-internal fun MainActivity.isMediaStoreUri(uri: Uri): Boolean {
+internal fun isMediaStoreUri(uri: Uri): Boolean {
         val authority = uri.authority ?: return false
         return authority == "media" ||
                authority.startsWith("media.") ||
@@ -61,7 +61,7 @@ internal fun MainActivity.resolveMediaStoreExt(uri: Uri, fallbackExt: String?):
         return fallbackExt ?: ""
     }
 
-internal fun MainActivity.extFromFileName(name: String): String {
+internal fun extFromFileName(name: String): String {
         return when {
             name.endsWith(".m4a") -> ".m4a"
             name.endsWith(".mp4") -> ".mp4"
@@ -81,7 +81,7 @@ internal fun MainActivity.extFromFileName(name: String): String {
         }
     }
 
-internal fun MainActivity.extFromMimeType(mime: String?): String {
+internal fun extFromMimeType(mime: String?): String {
         return when (mime) {
             "audio/mp4" -> ".m4a"
             "audio/aac" -> ".aac"

android/app/src/main/kotlin/com/zarz/spotiflac/NativeDownloadFinalizer.kt

@@ -869,7 +869,7 @@ object NativeDownloadFinalizer {
         check(!metadata.has("error")) { "ReplayGain verification failed: $metadata" }
         val isOpus = metadata.optString("audio_codec", "") == "opus"
         for (key in fields.keys()) {
-            // Opus stores only R128 gain, exposed by the Go reader as dB.
+            // Opus stores only R128 gain, exposed by the Rust reader as dB.
             if (isOpus && key.endsWith("_peak")) continue
             val expected = fields.optString(key, "").trim().removeSuffix("dB").trim().toDoubleOrNull()
             val actual = metadata.optString(key, "").trim().removeSuffix("dB").trim().toDoubleOrNull()

android/app/src/main/kotlin/com/zarz/spotiflac/SafDownloadHandler.kt

@@ -870,12 +870,4 @@ object SafDownloadHandler {
         val baseName = if (artistName.isNotBlank()) "$artistName - $trackName" else trackName
         return forceFilenameExt(baseName, outputExt)
     }
-
-    private fun errorJson(message: String): String {
-        val obj = JSONObject()
-        obj.put("success", false)
-        obj.put("error", message)
-        obj.put("message", message)
-        return obj.toString()
-    }
 }

ios/Runner/AppDelegate.swift

@@ -308,7 +308,7 @@ import UniformTypeIdentifiers
         switch call.method {
         case "beginBackgroundDownloadTask":
             downloadsActive = true
-            // Request the assertion before the long-running Go call starts.
+            // Request the assertion before the long-running backend call starts.
             // Waiting for applicationDidEnterBackground is too late: iOS may
             // suspend the process before the assertion is granted.
             beginBackgroundDownloadTask()
@@ -351,7 +351,7 @@ import UniformTypeIdentifiers
 
         DispatchQueue.global(qos: .userInitiated).async {
             do {
-                let response = try self.invokeGoMethod(call: call)
+                let response = try self.invokeOsMethod(call: call)
                 DispatchQueue.main.async {
                     result(response)
                 }
@@ -477,7 +477,7 @@ import UniformTypeIdentifiers
         }
     }
 
-    private func invokeGoMethod(call: FlutterMethodCall) throws -> Any? {
+    private func invokeOsMethod(call: FlutterMethodCall) throws -> Any? {
 
         switch call.method {
         case "getBackendImplementations":

ios/Runner/CoreBackend.swift

@@ -151,7 +151,7 @@ final class CoreDirectoryScope {
     deinit { close() }
 }
 
-/// Native migration boundary; each process selects one stateful backend.
+/// Native boundary to the Rust backend; each process holds one stateful instance.
 protocol CoreBackend {
     var implementation: String { get }
     func prepareApplicationArguments(method: String, arguments: Any?) throws -> Any?

ios/Runner/DownloadProgressSubscription.swift

@@ -129,7 +129,7 @@ final class DownloadProgressSubscription {
                 }
             }
             // Schedule from main so subscription identity is never accessed
-            // concurrently, and stop/relisten does not wait for an old Go call.
+            // concurrently, and stop/relisten does not wait for an old backend call.
             DispatchQueue.main.async { [weak self] in
                 guard let self, self.current === session, !session.isCancelled else { return }
                 DispatchQueue.main.asyncAfter(deadline: .now() + self.interval) { [weak self] in