feat(io): semantic styled-text model (Style, Span, StyledText)

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

Semesterprojekt/src/test/java/thb/jeanluc/adventure/io/text/StyledTextTest.java

@@ -0,0 +1,38 @@
+package thb.jeanluc.adventure.io.text;
+
+import org.junit.jupiter.api.Test;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+class StyledTextTest {
+
+    @Test
+    void builder_collectsSpansInOrder() {
+        StyledText t = StyledText.builder()
+                .plain("You take ").item("the brass lamp").plain(".").build();
+        assertThat(t.spans()).hasSize(3);
+        assertThat(t.spans().get(1).style()).isEqualTo(Style.ITEM);
+        assertThat(t.spans().get(1).text()).isEqualTo("the brass lamp");
+    }
+
+    @Test
+    void plainText_concatenatesAllSpanText() {
+        StyledText t = StyledText.builder()
+                .plain("You take ").item("the brass lamp").plain(".").build();
+        assertThat(t.plainText()).isEqualTo("You take the brass lamp.");
+    }
+
+    @Test
+    void of_createsSinglePlainSpan() {
+        StyledText t = StyledText.of("hello");
+        assertThat(t.spans()).hasSize(1);
+        assertThat(t.spans().getFirst().style()).isEqualTo(Style.PLAIN);
+        assertThat(t.plainText()).isEqualTo("hello");
+    }
+
+    @Test
+    void span_rejectsNulls() {
+        assertThatThrownBy(() -> new Span(null, Style.PLAIN))
+                .isInstanceOf(IllegalArgumentException.class);
+    }
+}