Mark some APIs experimental we aren't ready to commit to

nexus-sdk/src/main/java/io/nexusrpc/Serializer.java

@@ -6,6 +6,7 @@
 import org.jspecify.annotations.Nullable;
 
 /** Serializer used to convert values to bytes and vice-versa. */
+@Experimental
 public interface Serializer {
   /** Serialize the value as content. */
   Content serialize(@Nullable Object value);

nexus-sdk/src/main/java/io/nexusrpc/handler/Handler.java

@@ -1,8 +1,10 @@
 package io.nexusrpc.handler;
 
+import io.nexusrpc.Experimental;
 import io.nexusrpc.OperationException;
 
 /** Top-level handler for service calls. */
+@Experimental
 public interface Handler {
   /**
    * Handle the start of an operation. See {@link OperationHandler#start} for operation details.

nexus-sdk/src/main/java/io/nexusrpc/handler/HandlerInputContent.java

@@ -1,5 +1,6 @@
 package io.nexusrpc.handler;
 
+import io.nexusrpc.Experimental;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
@@ -9,6 +10,7 @@
 import org.jspecify.annotations.Nullable;
 
 /** Content that can be fixed or streaming for start operation input. */
+@Experimental
 public class HandlerInputContent {
   /** Create a builder for content. */
   public static Builder newBuilder() {

nexus-sdk/src/main/java/io/nexusrpc/handler/OperationHandler.java

@@ -1,5 +1,6 @@
 package io.nexusrpc.handler;
 
+import io.nexusrpc.Experimental;
 import io.nexusrpc.OperationException;
 import org.jspecify.annotations.Nullable;
 
@@ -13,6 +14,7 @@
  * @param <T> The parameter type of the operation. This can be {@link Void} for no parameter.
  * @param <R> the return type of the operation. This can be {@link Void} for no return.
  */
+@Experimental
 public interface OperationHandler<T, R> {
   /**
    * Create an operation handler for a synchronous operation backed by the given function. This

nexus-sdk/src/main/java/io/nexusrpc/handler/OperationMiddleware.java

@@ -1,6 +1,9 @@
 package io.nexusrpc.handler;
 
+import io.nexusrpc.Experimental;
+
 /** Middleware for intercepting operations. */
+@Experimental
 public interface OperationMiddleware {
 
   /** Intercepts the given operation. Called once for each operation invocation. */

nexus-sdk/src/main/java/io/nexusrpc/handler/ServiceHandler.java

@@ -5,6 +5,7 @@
 import org.jspecify.annotations.Nullable;
 
 /** Handler that delegates to service implementations. */
+@Experimental
 public class ServiceHandler implements Handler {
   /** Create a builder for a service handler. */
   public static Builder newBuilder() {