refactor(reader): FlatSegmentDecoder is package-private

Public but used nowhere outside the reader package; the layout
machinery (ScanIterator dispatch, VortexHandle impls) is its only
audience. VortexHandle.registry() javadoc no longer cross-refs the
now-hidden type.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: dfa1

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

@@ -23,7 +23,7 @@
 ///
 /// [ReadRegistry] is pure dispatch; this class owns all file-format knowledge:
 /// FlatBuffer parsing, buffer-offset arithmetic, and encoding-spec lookup.
-public final class FlatSegmentDecoder {
+final class FlatSegmentDecoder {
 
     /// Hard cap on array-node recursion depth. The encoded array tree nests through child nodes
     /// (validity, patches, run-ends, dictionary codes/values, …); a crafted or self-referential

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

@@ -48,8 +48,8 @@ public interface VortexHandle extends Closeable {
     /// Returns the [ReadRegistry] this handle was opened with.
     ///
     /// **Internal escape hatch.** Exposed for tooling
-    /// (e.g. the inspector's dictionary preview) that needs to decode an
-    /// internal subtree node directly via [io.github.dfa1.vortex.reader.FlatSegmentDecoder].
+    /// (e.g. the inspector's dictionary preview) that needs to resolve encoding
+    /// decoders for an internal subtree node.
     /// Not part of the supported stability contract; signatures may change
     /// without deprecation.
     ///