feat(io): welcome banner for game start

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Semesterprojekt/src/main/java/thb/jeanluc/adventure/io/text/Banner.java

@@ -0,0 +1,18 @@
+package thb.jeanluc.adventure.io.text;
+
+/** Builds styled banners for big moments (game start, finale). */
+public final class Banner {
+
+    private Banner() {
+    }
+
+    /** A framed, heading-styled welcome banner for the given title. */
+    public static StyledText welcome(String title) {
+        String bar = "=".repeat(Math.max(8, title.length() + 8));
+        return StyledText.builder()
+                .heading(bar + "\n")
+                .heading("    " + title + "\n")
+                .heading(bar)
+                .build();
+    }
+}