fix: restore SAF metadata fallback and preserve active transfers

Propagate filesystem errors from complete metadata reads so Android retries unreadable SAF descriptors through temporary files while accepting valid audio without tags.

Keep established segmented and chunked transfers outside the resolution allowance. Continue charging initial requests, retries, and progress callbacks, including during parallel native reads, while preserving stall cancellation.

Add metadata, transfer, and native fallback regression tests. Validated with Go tests, race detector, vet, Android tests, targeted Flutter tests, analyzer, and formatting checks.

android/app/src/test/kotlin/com/zarz/spotiflac/SafMetadataReadPolicyTest.kt

@@ -6,6 +6,14 @@ import org.junit.Assert.assertNull
 import org.junit.Test
 
 class SafMetadataReadPolicyTest {
+    @Test fun completeMetadataReadErrorUsesTemporaryCopy() {
+        val metadata = mapOf("lyrics" to "words", "comment" to "notes", "track_number" to 3)
+        assertEquals(metadata, readSafMetadataWithFallback(
+            directRead = { throw Exception("failed to read metadata: permission denied") },
+            fallbackRead = { metadata },
+        ))
+    }
+
     @Test fun fallbackFailureIsIsolatedToOneFile() {
         assertNull(readSafMetadataWithFallback<String>({ null }, { throw IllegalArgumentException("malformed metadata") }))
         assertEquals("next file", readSafMetadataWithFallback({ "next file" }, { error("copy") }))