fix: make Buffer::nth() positional so it stays correct after deletions

Buffer::nth() resolved the requested position by adding it to the first row key
and indexing the rows array by that computed key. That assumes contiguous keys,
but Buffer::delete() removes rows with unset() without reindexing, leaving gaps.
After a deletion nth() therefore returned the wrong record (or an empty array for
a valid position, or a record for an out-of-range position). Resolve the position
against the record order with array_values() so nth() matches getRecords() and
ResultSet::nth().

src/Buffer.php

@@ -335,16 +335,11 @@ final class Buffer implements TabularData
     private function nthRow(int $nth, string $method): array
     {
         -1 < $nth || throw InvalidArgument::dueToInvalidRecordOffset($nth, $method);
-        if (null === ($first = $this->firstOffset())) {
-            return [];
-        }
 
-        $offset = $first + $nth;
-        if (!array_key_exists($offset, $this->rows)) {
-            return [];
-        }
-
-        return $this->rows[$nth + $first];
+        // $nth is a positional index (like ResultSet::nth), so it must be
+        // resolved against the record order rather than the row keys, which
+        // may contain gaps after records have been deleted.
+        return array_values($this->rows)[$nth] ?? [];
     }
 
     public function fetchColumn(int|string $index = 0): Iterator

src/BufferTest.php

@@ -285,6 +285,24 @@ final class BufferTest extends TestCase
         self::assertSame(['date' => '2025-02-08', 'temperature' => '3', 'place' => 'Berkeley'], $buffer->nth(0));
     }
 
+    #[Test]
+    public function it_returns_records_by_positional_offset_after_a_middle_record_is_deleted(): void
+    {
+        $buffer = new Buffer(['id', 'name']);
+        $buffer->insert(['1', 'a'], ['2', 'b'], ['3', 'c'], ['4', 'd'], ['5', 'e']);
+
+        // Delete the second record (id = 2), which leaves a gap in the row keys.
+        self::assertSame(1, $buffer->delete(fn (array $row): bool => $row['id'] === '2'));
+        self::assertSame(4, $buffer->recordCount());
+
+        // nth() is positional and must stay consistent with getRecords().
+        self::assertSame(['id' => '1', 'name' => 'a'], $buffer->nth(0));
+        self::assertSame(['id' => '3', 'name' => 'c'], $buffer->nth(1));
+        self::assertSame(['id' => '4', 'name' => 'd'], $buffer->nth(2));
+        self::assertSame(['id' => '5', 'name' => 'e'], $buffer->nth(3));
+        self::assertSame([], $buffer->nth(4));
+    }
+
     #[Test]
     public function it_can_not_insert_invalid_records(): void
     {