review: clarify isUnsigned javadoc; pin signed-I8 rendering

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dfa1

csv/src/main/java/io/github/dfa1/vortex/csv/CsvExporter.java

@@ -166,7 +166,8 @@ private static String cellValue(Array arr, long rowIdx) {
 
     /// Whether the array's dtype is an unsigned integer, so high-half values must render as
     /// unsigned decimal rather than the two's-complement negative that the raw signed getter
-    /// returns. A [MaskedArray]'s inner array carries the same ptype, so recursing is safe.
+    /// returns. Callers only pass leaf value arrays: the [MaskedArray] arm of `cellValue`
+    /// recurses into the inner array before any integer arm is reached.
     private static boolean isUnsigned(Array arr) {
         return arr.dtype() instanceof DType.Primitive p && p.ptype().isUnsigned();
     }

csv/src/test/java/io/github/dfa1/vortex/csv/CsvExporterTest.java

@@ -143,6 +143,29 @@ void rendersNullableU8HighHalfAsUnsigned(@TempDir Path tmp) throws Exception {
         assertThat(result).containsExactly("magnesium", "132", "", "162");
     }
 
+    @Test
+    void keepsSignedI8Negative(@TempDir Path tmp) throws Exception {
+        // Given a SIGNED I8 column with a negative value — pins that the unsigned-rendering
+        // fix did not widen signed bytes: -5 must stay "-5", not become "251"
+        Path vortex = tmp.resolve("i8.vortex");
+        DType.Struct schema = new DType.Struct(
+                List.of(ColumnName.of("delta")),
+                List.of(new DType.Primitive(PType.I8, false)),
+                false);
+        try (FileChannel ch = FileChannel.open(vortex, StandardOpenOption.CREATE, StandardOpenOption.WRITE);
+             VortexWriter writer = VortexWriter.create(ch, schema, WriteOptions.defaults())) {
+            writer.writeChunk(Map.of(ColumnName.of("delta"), new byte[]{(byte) -5, (byte) 7}));
+        }
+        Path csv = tmp.resolve("out.csv");
+
+        // When
+        CsvExporter.exportCsv(vortex, csv);
+
+        // Then
+        List<String> result = Files.readAllLines(csv);
+        assertThat(result).containsExactly("delta", "-5", "7");
+    }
+
     @Test
     void rendersU16HighHalfAsUnsigned(@TempDir Path tmp) throws Exception {
         // Given a non-nullable U16 column. 40000 is 0x9C40, which is -25536 as a signed short.