test(reader): cover overflowed-zone null path in ZoneReducer

Add an overflowed-zone case to ZoneReducerTest: zone maps enabled but one
I64 chunk's SUM overflows (Math.addExact drops it), so the zone-map table
is present yet a zone carries no usable sum — exercising the fold's bail
path distinctly from the absent-table case.

Also add WriteOptions.withZoneMaps(boolean), mirroring withGlobalDict /
withZstd, so the no-zone-map test reads as
WriteOptions.defaults().withZoneMaps(false) instead of a brittle
positional constructor call.

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

Author: dfa1

writer/src/main/java/io/github/dfa1/vortex/writer/WriteOptions.java

@@ -37,6 +37,14 @@ public static WriteOptions cascading(int depth) {
         return new WriteOptions(65_536, true, 0.90, depth, true, false);
     }
 
+    /// Returns a copy of these options with zone-map statistics set to `enabled`.
+    ///
+    /// @param enabled `true` to write per-chunk min/max/sum statistics for zone-map pruning
+    /// @return a new `WriteOptions` with the zone-map flag updated
+    public WriteOptions withZoneMaps(boolean enabled) {
+        return new WriteOptions(chunkSize, enabled, compressionRatioThreshold, allowedCascading, globalDict, enableZstd);
+    }
+
     /// Returns a copy of these options with global dictionary encoding set to `enabled`.
     ///
     /// @param enabled `true` to enable global dictionary encoding across chunks

writer/src/test/java/io/github/dfa1/vortex/writer/ZoneReducerTest.java

@@ -81,7 +81,7 @@ void floatSumFoldsZonesToDouble(@TempDir Path tmp) throws IOException {
     void noZoneMapYieldsNull(@TempDir Path tmp) throws IOException {
         // Given — zone maps disabled, so no per-zone SUM exists to fold
         Path file = tmp.resolve("nostats.vtx");
-        WriteOptions noZoneMaps = new WriteOptions(65_536, false, 0.90, 0, true, false);
+        WriteOptions noZoneMaps = WriteOptions.defaults().withZoneMaps(false);
         try (var ch = FileChannel.open(file, StandardOpenOption.CREATE, StandardOpenOption.WRITE);
              var w = VortexWriter.create(ch, I64_SCHEMA, noZoneMaps)) {
             w.writeChunk(Map.of("id", range(1L, 50L)));
@@ -95,4 +95,25 @@ void noZoneMapYieldsNull(@TempDir Path tmp) throws IOException {
             assertThat(result).isNull();
         }
     }
+
+    @Test
+    void overflowedZoneYieldsNull(@TempDir Path tmp) throws IOException {
+        // Given — zone maps ON, but one zone's I64 SUM overflows long (Math.addExact in the writer
+        // drops it), so a zone map exists yet a zone carries no usable sum. Distinct from
+        // noZoneMapYieldsNull: there the table is absent; here it is present but incomplete.
+        Path file = tmp.resolve("overflow.vtx");
+        try (var ch = FileChannel.open(file, StandardOpenOption.CREATE, StandardOpenOption.WRITE);
+             var w = VortexWriter.create(ch, I64_SCHEMA, WriteOptions.defaults())) {
+            w.writeChunk(Map.of("id", range(1L, 50L)));                                // sums fine
+            w.writeChunk(Map.of("id", new long[]{Long.MAX_VALUE, Long.MAX_VALUE}));    // overflows
+        }
+
+        // When
+        try (VortexReader reader = VortexReader.open(file, registry())) {
+            Number result = new ZoneReducer(reader).sum("id");
+
+            // Then — one unusable zone forces the whole fold to bail; no partial sum returned
+            assertThat(result).isNull();
+        }
+    }
 }