docs(core): vortex.flat is a layout id, not an encoding id

The EncodingId javadoc examples used "vortex.flat", which lives in
the footer's layoutSpecs table (Layout.FLAT), never in arraySpecs;
the canonical flat encoding id is "vortex.primitive".

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

Author: dfa1

core/src/main/java/io/github/dfa1/vortex/core/model/EncodingId.java

@@ -9,15 +9,15 @@
 
 /// Identity of an array encoding — either a spec [WellKnown] constant or a third-party [Custom] id.
 ///
-/// The wire representation is always a string (e.g. `"vortex.flat"`); [#parse(String)] maps any
+/// The wire representation is always a string (e.g. `"vortex.primitive"`); [#parse(String)] maps any
 /// such string to a typed value, and [#id()] recovers the wire string from a typed value.
 ///
 /// Extends [Serializable] so a [Custom] or [WellKnown] carried on a
 /// [io.github.dfa1.vortex.core.error.VortexException] survives serialization, matching the prior
 /// enum's implicit serializability.
 public sealed interface EncodingId extends Serializable permits EncodingId.WellKnown, EncodingId.Custom {
 
-    /// Returns the wire string of this encoding id (e.g. `"vortex.flat"`).
+    /// Returns the wire string of this encoding id (e.g. `"vortex.primitive"`).
     ///
     /// @return the wire string of this encoding id
     String id();