refactor(reader)!: drop dead EmptyArray, enforce dtype→family invariant

EmptyArray was never constructed in production — empties are represented
as zero-length typed arrays in their own family (LazyConstant*, Materialized*
len 0). It only occupied a slot in sealed Array's permits, adding a dead case
to exhaustive switches, and broke the dtype→family invariant (EmptyArray(I64)
is not a LongArray, so `case LongArray` consumers would miss it).

Remove the type and its permit. The lone user — a universal stub decoder in
VortexReaderTest — now returns a zero-length UnknownArray.

BREAKING CHANGE: EmptyArray removed from the sealed Array permits.

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

Author: dfa1

reader/src/main/java/io/github/dfa1/vortex/reader/array/Array.java

@@ -9,7 +9,7 @@
 /// Buffers are `MemorySegment` slices backed by the memory-mapped file; lifetime
 /// is tied to the `VortexFile`'s Arena.
 public sealed interface Array
-        permits BoolArray, ByteArray, DecimalArray, DoubleArray, EmptyArray, FixedSizeListArray,
+        permits BoolArray, ByteArray, DecimalArray, DoubleArray, FixedSizeListArray,
                         Float16Array, FloatArray, GenericArray, IntArray, ListArray, ListViewArray,
                         LongArray, MaskedArray, NullArray, ShortArray, StructArray, UnknownArray,
                         VarBinArray, VariantArray {

reader/src/main/java/io/github/dfa1/vortex/reader/array/EmptyArray.java

@@ -4,7 +4,6 @@
 import io.github.dfa1.vortex.core.VortexException;
 import io.github.dfa1.vortex.core.VortexFormat;
 import io.github.dfa1.vortex.reader.array.Array;
-import io.github.dfa1.vortex.reader.array.EmptyArray;
 import io.github.dfa1.vortex.reader.array.UnknownArray;
 import io.github.dfa1.vortex.reader.decode.DecodeContext;
 import io.github.dfa1.vortex.reader.decode.EncodingDecoder;
@@ -16,6 +15,7 @@
 import org.junit.jupiter.params.provider.ValueSource;
 
 import java.io.IOException;
+import java.lang.foreign.MemorySegment;
 import java.net.URISyntaxException;
 import java.nio.ByteOrder;
 import java.nio.file.Files;
@@ -44,7 +44,10 @@ public boolean accepts(DType dtype) {
 
                 @Override
                 public Array decode(DecodeContext ctx) {
-                    return EmptyArray.of(ctx.dtype());
+                    // Generic zero-length stand-in: scan chunk row count comes from the
+                    // layout, so the leaf array's contents are irrelevant to this test.
+                    return new UnknownArray("stub", ctx.dtype(), 0, null,
+                            new MemorySegment[0], new Array[0]);
                 }
             });
         }