feat(reader): aggregate null_count across chunks in columnStats

Per-chunk null_count now flows into columnStats(): aggregateStats sums
counts across flats, reporting the total only when every chunk carries
one (a single missing count makes the column total unknown -> null).
Also fixes the early-return guard, which dropped null_count for all-null
columns that have no min/max.

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

Author: dfa1

reader/src/main/java/io/github/dfa1/vortex/reader/VortexReader.java

@@ -181,6 +181,10 @@ public Map<String, ArrayStats> columnStats() {
     private ArrayStats aggregateStats(List<Layout> flats) {
         Object globalMin = null;
         Object globalMax = null;
+        // Sum is meaningful only when every chunk carries a null count; one missing makes the
+        // column total unknown (null), so don't report a partial sum.
+        long totalNullCount = 0L;
+        boolean allHaveNullCount = !flats.isEmpty();
         for (Layout flat : flats) {
             ArrayStats s = readFlatStats(flat);
             if (s.min() != null) {
@@ -189,11 +193,17 @@ private ArrayStats aggregateStats(List<Layout> flats) {
             if (s.max() != null) {
                 globalMax = globalMax == null ? s.max() : maxOf(globalMax, s.max());
             }
+            if (s.nullCount() != null) {
+                totalNullCount += s.nullCount();
+            } else {
+                allHaveNullCount = false;
+            }
         }
-        if (globalMin == null && globalMax == null) {
+        Long nullCount = allHaveNullCount ? totalNullCount : null;
+        if (globalMin == null && globalMax == null && nullCount == null) {
             return ArrayStats.empty();
         }
-        return new ArrayStats(globalMin, globalMax, null, null, null, null);
+        return new ArrayStats(globalMin, globalMax, null, nullCount, null, null);
     }
 
     private ArrayStats readFlatStats(Layout flat) {

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

@@ -78,4 +78,20 @@ void noFilter_keepsAllChunks() throws IOException {
         // Given / When / Then all three chunks survive
         assertThat(scanRowCounts(write(), null)).containsExactly(3L, 2L, 4L);
     }
+
+    @Test
+    void columnStats_sumsNullCountAcrossChunks() throws IOException {
+        // Given chunks carry 0, 1 and 4 nulls; every chunk records a null count so the
+        // column total is known (0 + 1 + 4 = 5) rather than dropped to unknown
+        Path file = write();
+
+        // When
+        Long result;
+        try (VortexReader reader = VortexReader.open(file)) {
+            result = reader.columnStats().get("v").nullCount();
+        }
+
+        // Then
+        assertThat(result).isEqualTo(5L);
+    }
 }