refactor(reader): drop dead registry field from ScanIterator

The field was stored but never called — all encoding dispatch
goes through VortexHandle.decodeFlatSegment → FlatSegmentDecoder,
which already holds its own ReadRegistry. Remove the parameter
from the constructor and both call sites.

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

Author: dfa1

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

@@ -84,7 +84,6 @@ public final class ScanIterator implements Iterator<Chunk>, AutoCloseable {
     private static final ValueLayout.OfLong LE_LONG = ValueLayout.JAVA_LONG_UNALIGNED.withOrder(ByteOrder.LITTLE_ENDIAN);
 
     private final VortexHandle file;
-    private final ReadRegistry registry;
     private final ScanOptions options;
 
     private List<ChunkSpec> chunks;
@@ -96,9 +95,8 @@ public final class ScanIterator implements Iterator<Chunk>, AutoCloseable {
     private Chunk openChunk;
     private boolean closed;
 
-    public ScanIterator(VortexHandle file, ReadRegistry registry, ScanOptions options) {
+    public ScanIterator(VortexHandle file, ScanOptions options) {
         this.file = file;
-        this.registry = registry;
         this.options = options;
     }
 

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

@@ -278,7 +278,7 @@ public MemorySegment rawSegment(SegmentSpec spec) {
 
     @Override
     public ScanIterator scan(ScanOptions options) {
-        return new ScanIterator(this, registry, options);
+        return new ScanIterator(this, options);
     }
 
     @Override

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

@@ -153,7 +153,7 @@ public long fileSize() {
 
     @Override
     public ScanIterator scan(ScanOptions options) {
-        return new ScanIterator(this, registry, options);
+        return new ScanIterator(this, options);
     }
 
     @Override