Add sealed module: modelling domains with sealed types and exhaustive switch

Payment model, permits and the three modifiers, default-branch trap, generic and recursive hierarchies, visitor comparison, run-time enforcement, modules. Transcripts from JDK 21, 25 and 27.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01TF9JWFvJSNm6HVzswzZU5a

sealed/sep/Main.java

@@ -0,0 +1,7 @@
+public class Main {
+    public static void main(String[] args) throws Exception {
+        System.out.println("loading Card:  " + Class.forName("Card").getSimpleName());
+        System.out.println("loading Hack...");
+        System.out.println(Class.forName("Hack").getSimpleName());
+    }
+}

sealed/sep/evil/Card.java

@@ -0,0 +1 @@
+public final class Card implements Payment {}

sealed/sep/evil/Hack.java

@@ -0,0 +1 @@
+public final class Hack implements Payment {}

sealed/sep/evil/Payment.java

@@ -0,0 +1 @@
+public sealed interface Payment permits Card, Hack {}

sealed/sep/v1/Card.java

@@ -0,0 +1 @@
+public final class Card implements Payment {}

sealed/sep/v1/Payment.java

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+public sealed interface Payment permits Card {}