fix: use VortexException for missing validity child in MaskedArray

IOOBE signals programmer bugs; missing validity child when AllValid
was encoded is a file-data inconsistency — callers must distinguish.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dfa1

core/src/main/java/io/github/dfa1/vortex/core/array/MaskedArray.java

@@ -1,6 +1,7 @@
 package io.github.dfa1.vortex.core.array;
 
 import io.github.dfa1.vortex.core.DType;
+import io.github.dfa1.vortex.core.VortexException;
 
 import java.lang.foreign.MemorySegment;
 
@@ -47,7 +48,7 @@ public Array child(int i) {
 			case 0 -> child;
 			case 1 -> {
 				if (validity == null) {
-					throw new IndexOutOfBoundsException("no validity child (AllValid)");
+					throw new VortexException("no validity child: masked array was encoded as AllValid");
 				}
 				yield validity;
 			}