Add patterns module: pattern matching from JDK 21 to 27

Type, record and primitive patterns, guards, sealed exhaustiveness, MatchException via separate compilation, and eight captured transcripts.

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

patterns/src/SealedExhaustive.java

@@ -0,0 +1,21 @@
+public class SealedExhaustive {
+    sealed interface Shape permits Circle, Square, Rect {}
+    record Circle(double r) implements Shape {}
+    record Square(double side) implements Shape {}
+    record Rect(double w, double h) implements Shape {}
+
+    // No default: the compiler checks that every permitted subtype is covered.
+    static double area(Shape s) {
+        return switch (s) {
+            case Circle(double r) -> Math.PI * r * r;
+            case Square(double side) -> side * side;
+            case Rect(double w, double h) -> w * h;
+        };
+    }
+
+    public static void main(String[] args) {
+        for (Shape s : new Shape[]{new Circle(1), new Square(2), new Rect(2, 3)}) {
+            System.out.printf("%s -> %.2f%n", s, area(s));
+        }
+    }
+}