Validate preview sessions against real classpath and module-path fixtures

designer-preview-api/src/main/java/de/neitzel/fxdesigner/preview/PreviewSession.java

@@ -0,0 +1,23 @@
+package de.neitzel.fxdesigner.preview;
+
+import java.util.concurrent.*;
+
+/** Owned example state. The NeitzelLib adapter requires C to be an ApplicationContext. */
+public interface PreviewSession<M, C> extends AutoCloseable {
+  M model();
+
+  C context();
+
+  /**
+   * Called after View loading/CSS/layout, on FX. Never block FX; complete asynchronously if needed.
+   */
+  default CompletionStage<Void> ready() {
+    return CompletableFuture.completedFuture(null);
+  }
+
+  /**
+   * Release owned services/timers/listeners. Called once on FX, including load/readiness failure.
+   */
+  @Override
+  void close() throws Exception;
+}