Add http3 module: HTTP/3 in the Java HTTP Client (JEP 517), opt-in, discovery modes, fallback timings, proxy downgrade and a packet-loss comparison

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01KqJyCidz3ZgRyHABv2GVJh

http3/src/OptIn.java

@@ -0,0 +1,22 @@
+import java.net.URI;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
+
+/** The JEP 517 opt-in, client-wide or per request. Prints the version every response really used. */
+public class OptIn {
+    public static void main(String[] args) throws Exception {
+        URI uri = URI.create(args[0]);
+        String where = args.length > 1 ? args[1] : "client";       // "client" or "request"
+        var builder = Tls.builder();
+        if (where.equals("client")) builder.version(HttpClient.Version.HTTP_3);
+        var client = builder.build();
+        var req = HttpRequest.newBuilder(uri);
+        if (where.equals("request")) req.version(HttpClient.Version.HTTP_3);
+        for (int i = 1; i <= 4; i++) {
+            var r = client.send(req.build(), HttpResponse.BodyHandlers.ofString());
+            System.out.println(where + "-level HTTP_3, request " + i + ": " + r.version());
+        }
+        client.close();
+    }
+}