fix(storage): retry index file creation while the target is busy

A just-deleted index file can still be held by the system, so
createNewFile() may fail right after deletion. Retry a few times
with a short delay, waiting for the old file to disappear first.

app/src/main/java/com/example/research/common/util/FileStorageManager.kt

@@ -19,6 +19,8 @@ class FileStorageManager {
         private const val DELETE_WAIT_INTERVAL = 50L
         private const val MAX_DELETE_WAIT_COUNT = 10
         private const val DEFAULT_MAX_RETRIES = 5
+        private const val CREATE_RETRY_DELAY = 100L
+        private const val MAX_CREATE_RETRIES = 3
         private const val TAG = "FileStorageManager"
     }
 
@@ -87,6 +89,7 @@ class FileStorageManager {
                         Log.e(TAG, "Could not delete existing file: $fileName")
                         return@withContext null
                     }
+                    awaitGone(targetFile::exists)
                 } else {
                     return@withContext Uri.fromFile(targetFile)
                 }
@@ -95,7 +98,18 @@ class FileStorageManager {
             if (tempBinFile.exists()) {
                 tempBinFile.delete()
             }
-            if (targetFile.createNewFile()) {
+            var created = targetFile.createNewFile()
+            var createAttempt = 0
+            while (!created && createAttempt < MAX_CREATE_RETRIES) {
+                delay(CREATE_RETRY_DELAY.milliseconds)
+                if (targetFile.exists()) {
+                    targetFile.delete()
+                    awaitGone(targetFile::exists)
+                }
+                created = targetFile.createNewFile()
+                createAttempt++
+            }
+            if (created) {
                 Uri.fromFile(targetFile)
             } else {
                 Log.e(TAG, "Failed to create file: $fileName")
@@ -116,7 +130,7 @@ class FileStorageManager {
         for (attempt in 1..maxRetries) {
             try {
                 if (file.delete()) {
-                    waitForFileDeletion(file)
+                    awaitGone(file::exists)
                     if (!file.exists()) {
                         return@withContext true
                     }
@@ -141,9 +155,10 @@ class FileStorageManager {
         }
     }
 
-    private suspend fun waitForFileDeletion(file: DocumentFile) {
+    // FUSE-backed storage keeps a just-deleted file visible to stat/open for a short window
+    private suspend fun awaitGone(exists: () -> Boolean) {
         var waitCount = 0
-        while (file.exists() && waitCount < MAX_DELETE_WAIT_COUNT) {
+        while (exists() && waitCount < MAX_DELETE_WAIT_COUNT) {
             delay(DELETE_WAIT_INTERVAL.milliseconds)
             waitCount++
         }