Add 100-file volume test (subfolders + non-ASCII, 0 failures, no re-sync loop)

Verifies the engine handles 100+ files in one sync without failures and that a
follow-up sync is a clean no-op (no phantom re-uploads at volume).

app/src/androidTest/kotlin/com/syncflow/FullSyncEngineTest.kt

@@ -253,6 +253,23 @@ class FullSyncEngineTest {
         assertTrue("my photo.txt" in names)
     }
 
+    // ── 13c. Volume: 100+ files (incl. subfolders & non-ASCII) upload, 0 fails ─
+    @Test fun volume_hundredFiles_allUploadNoFailures() = runBlocking {
+        val (pair, local, remote) = newPair("vol100", SyncDirection.UPLOAD_ONLY, DeleteBehavior.KEEP)
+        repeat(100) { i -> write(local, "f_%03d.txt".format(i), "payload $i ".repeat(30)) }
+        write(local, "sub/nested_a.txt", "n1")
+        write(local, "sub/deep/nested_b.txt", "n2")
+        write(local, "naïve café.txt", "accented")
+        val r = sync(pair)
+        assertEquals("no file may fail under volume", 0, r.failedFiles)
+        assertEquals("all 103 files upload", 103, r.uploaded)
+        assertEquals("100 flat files present on cloud", 100, remoteNames(remote).count { it.startsWith("f_") })
+        assertTrue("non-ASCII name present too", "naïve café.txt" in remoteNames(remote))
+        // re-sync is a clean no-op (no phantom re-uploads / loops at volume)
+        val r2 = sync(pair)
+        assertEquals(0, r2.uploaded); assertEquals(0, r2.deleted); assertEquals(0, r2.failedFiles)
+    }
+
     // ── 14. Content integrity: binary-ish bytes round-trip exactly ────────────
     @Test fun contentIntegrity_roundTrip() = runBlocking {
         val (pair, local, remote) = newPair("integrity", SyncDirection.TWO_WAY)