fix(reader): reset slab allocator between chunks

SlicingAllocator never resets, so decode allocations accumulated
across all chunks until the 4 MB slab overflowed with
IndexOutOfBoundsException. Reset by recreating the allocator from
the same slab before each chunk decode.

Author: dfa1

reader/src/main/java/io/github/dfa1/vortex/scan/ScanIterator.java

@@ -134,6 +134,7 @@ public boolean hasNext() {
 				continue;
 			}
 
+			chunkAlloc = SegmentAllocator.slicingAllocator(chunkSlab);
 			current = new ScanResult(chunk.rowCount(), buildColumnMap(chunk));
 			rowsReturned += chunk.rowCount();
 			return true;