Pre-pad new columns for existing rows

QwpTableBuffer.createColumn() created new column buffers with
size = 0 regardless of how many rows were already committed.
When the caller added a value for the current row, it landed at
position 0 (the first row's slot). The subsequent nextRow() call
padded nulls after the value, misaligning the column data with
all other columns.

The fix pre-pads the new column with nulls for all committed
rows so the next value lands at the correct row position.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mtopolnik

core/src/main/java/io/questdb/client/cutlass/qwp/protocol/QwpTableBuffer.java

@@ -351,6 +351,11 @@ private ColumnBuffer createColumn(CharSequence name, byte type, boolean nullable
             fastColumns = newArr;
         }
         fastColumns[index] = col;
+        // Pre-pad with nulls for already-committed rows so the next
+        // value the caller adds lands at the correct row position.
+        for (int r = 0; r < rowCount; r++) {
+            col.addNull();
+        }
         schemaHashComputed = false;
         columnDefsCacheValid = false;
         return col;