refactor(android): move SAF scan checkpointing out of MainActivitySafScan.kt

The resumable-scan checkpoint helpers (state file path, load,
reconcile against written rows, row count and output repair) are a
self-contained unit, so they move verbatim into SafScanCheckpoint.kt.
They were file-private and become internal because scanSafTree stays
in MainActivitySafScan.kt. The checkpoint file name, its line format
and the reconciled NDJSON keys are unchanged.

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

@@ -12,7 +12,6 @@ import org.json.JSONObject
 import java.io.File
 import java.io.FileOutputStream
 import java.io.IOException
-import java.io.RandomAccessFile
 import java.security.MessageDigest
 import java.util.Locale
 import java.util.concurrent.ExecutionException
@@ -91,92 +90,6 @@ internal fun MainActivity.loadExistingFilesFromSnapshot(snapshotPath: String): M
         return result
     }
 
-private fun safScanCheckpointPath(outputPath: String): String = "$outputPath.state"
-
-private fun loadSafScanCheckpoint(path: String): MutableMap<String, Long> {
-    val checkpoint = File(path)
-    if (!checkpoint.exists()) return mutableMapOf()
-    val entries = mutableMapOf<String, Long>()
-    try {
-        checkpoint.forEachLine(Charsets.UTF_8) { line ->
-            val separator = line.indexOf('\t')
-            if (separator <= 0) return@forEachLine
-            val uri = line.substring(0, separator)
-            val modified = line.substring(separator + 1).toLongOrNull() ?: return@forEachLine
-            entries[uri] = modified
-        }
-    } catch (e: Exception) {
-        android.util.Log.w("SpotiFLAC", "SAF scan: failed reading checkpoint: ${e.message}")
-    }
-    return entries
-}
-
-private fun reconcileSafScanCheckpoint(
-    outputPath: String,
-    loaded: MutableMap<String, Long>,
-): MutableMap<String, Long> {
-    val output = File(outputPath)
-    if (!output.exists()) return loaded
-
-    val outputEntries = mutableMapOf<String, Long>()
-    try {
-        output.forEachLine(Charsets.UTF_8) { line ->
-            if (line.isBlank()) return@forEachLine
-            try {
-                val obj = JSONObject(line)
-                val filePath = obj.optString("filePath", "").trim()
-                if (filePath.isNotBlank()) {
-                    val modified = obj.optLong("fileModTime", 0L)
-                    val cueMarker = filePath.indexOf("#track")
-                    val key = if (cueMarker > 0) filePath.substring(0, cueMarker) else filePath
-                    outputEntries[key] = modified
-                }
-            } catch (_: Exception) {}
-        }
-    } catch (e: Exception) {
-        android.util.Log.w("SpotiFLAC", "SAF scan: failed reconciling checkpoint: ${e.message}")
-        return loaded
-    }
-
-    loaded.clear()
-    loaded.putAll(outputEntries)
-    return loaded
-}
-
-private fun countSafScanRows(path: String): Int {
-    val output = File(path)
-    if (!output.exists()) return 0
-    return try {
-        output.useLines(Charsets.UTF_8) { lines -> lines.count { it.isNotBlank() } }
-    } catch (e: Exception) {
-        android.util.Log.w("SpotiFLAC", "SAF scan: failed counting existing rows: ${e.message}")
-        0
-    }
-}
-
-private fun repairSafScanOutput(path: String) {
-    val output = File(path)
-    if (!output.exists()) return
-    try {
-        RandomAccessFile(output, "rw").use { file ->
-            var position = file.length() - 1
-            while (position >= 0) {
-                file.seek(position)
-                if (file.readByte().toInt() == '\n'.code) {
-                    file.setLength(position + 1)
-                    return
-                }
-                position--
-            }
-            // No complete line survived the interruption.
-            file.setLength(0)
-        }
-    } catch (e: Exception) {
-        android.util.Log.w("SpotiFLAC", "SAF scan: failed repairing output: ${e.message}")
-    }
-}
-
-
     /**
      * Extract the audio filename referenced by a CUE sheet file.
      * Reads the FILE "name" TYPE line from the .cue text.

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

@@ -0,0 +1,94 @@
+package com.zarz.spotiflac
+
+import org.json.JSONObject
+import java.io.File
+import java.io.RandomAccessFile
+
+// Resumable SAF scan state: the `<output>.state` checkpoint beside the NDJSON
+// output, its reconciliation with the rows already written, and output repair
+// after an interrupted scan.
+
+internal fun safScanCheckpointPath(outputPath: String): String = "$outputPath.state"
+
+internal fun loadSafScanCheckpoint(path: String): MutableMap<String, Long> {
+    val checkpoint = File(path)
+    if (!checkpoint.exists()) return mutableMapOf()
+    val entries = mutableMapOf<String, Long>()
+    try {
+        checkpoint.forEachLine(Charsets.UTF_8) { line ->
+            val separator = line.indexOf('\t')
+            if (separator <= 0) return@forEachLine
+            val uri = line.substring(0, separator)
+            val modified = line.substring(separator + 1).toLongOrNull() ?: return@forEachLine
+            entries[uri] = modified
+        }
+    } catch (e: Exception) {
+        android.util.Log.w("SpotiFLAC", "SAF scan: failed reading checkpoint: ${e.message}")
+    }
+    return entries
+}
+
+internal fun reconcileSafScanCheckpoint(
+    outputPath: String,
+    loaded: MutableMap<String, Long>,
+): MutableMap<String, Long> {
+    val output = File(outputPath)
+    if (!output.exists()) return loaded
+
+    val outputEntries = mutableMapOf<String, Long>()
+    try {
+        output.forEachLine(Charsets.UTF_8) { line ->
+            if (line.isBlank()) return@forEachLine
+            try {
+                val obj = JSONObject(line)
+                val filePath = obj.optString("filePath", "").trim()
+                if (filePath.isNotBlank()) {
+                    val modified = obj.optLong("fileModTime", 0L)
+                    val cueMarker = filePath.indexOf("#track")
+                    val key = if (cueMarker > 0) filePath.substring(0, cueMarker) else filePath
+                    outputEntries[key] = modified
+                }
+            } catch (_: Exception) {}
+        }
+    } catch (e: Exception) {
+        android.util.Log.w("SpotiFLAC", "SAF scan: failed reconciling checkpoint: ${e.message}")
+        return loaded
+    }
+
+    loaded.clear()
+    loaded.putAll(outputEntries)
+    return loaded
+}
+
+internal fun countSafScanRows(path: String): Int {
+    val output = File(path)
+    if (!output.exists()) return 0
+    return try {
+        output.useLines(Charsets.UTF_8) { lines -> lines.count { it.isNotBlank() } }
+    } catch (e: Exception) {
+        android.util.Log.w("SpotiFLAC", "SAF scan: failed counting existing rows: ${e.message}")
+        0
+    }
+}
+
+internal fun repairSafScanOutput(path: String) {
+    val output = File(path)
+    if (!output.exists()) return
+    try {
+        RandomAccessFile(output, "rw").use { file ->
+            var position = file.length() - 1
+            while (position >= 0) {
+                file.seek(position)
+                if (file.readByte().toInt() == '\n'.code) {
+                    file.setLength(position + 1)
+                    return
+                }
+                position--
+            }
+            // No complete line survived the interruption.
+            file.setLength(0)
+        }
+    } catch (e: Exception) {
+        android.util.Log.w("SpotiFLAC", "SAF scan: failed repairing output: ${e.message}")
+    }
+}