refactor: annotate serialVersionUID with @serial

The three exception types declare `serialVersionUID`; mark each with
`java.io.Serial` so the compiler validates the serialization contract and the
intent is explicit (clears the IDE/Sonar "can be annotated with @serial" hint).

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

Author: dfa1

core/src/main/java/io/github/dfa1/vortex/core/error/VortexException.java

@@ -2,6 +2,7 @@
 
 import io.github.dfa1.vortex.core.model.EncodingId;
 
+import java.io.Serial;
 import java.util.Optional;
 
 /// Unrecoverable Vortex error: malformed file, unsupported feature, or encoding failure.
@@ -15,6 +16,7 @@
 /// for recovery logic.
 public final class VortexException extends RuntimeException {
 
+    @Serial
     private static final long serialVersionUID = 1L;
 
     /// The encoding that raised this exception, or `null` if not attributed to a specific encoding.

fbs-gen/src/main/java/io/github/dfa1/vortex/fbsgen/FbsParseException.java

@@ -1,8 +1,11 @@
 package io.github.dfa1.vortex.fbsgen;
 
+import java.io.Serial;
+
 /// Thrown when a `.fbs` schema cannot be lexed or parsed.
 public final class FbsParseException extends RuntimeException {
 
+    @Serial
     private static final long serialVersionUID = 1L;
 
     /// @param message human-readable diagnostic, ideally with a 1-based line number

proto-gen/src/main/java/io/github/dfa1/vortex/protogen/ProtoParseException.java

@@ -1,8 +1,11 @@
 package io.github.dfa1.vortex.protogen;
 
+import java.io.Serial;
+
 /// Thrown when `.proto` source is malformed or uses unsupported syntax.
 public final class ProtoParseException extends RuntimeException {
 
+    @Serial
     private static final long serialVersionUID = 1L;
 
     /// @param message diagnostic