fix: use patched Forge client jar instead of vanilla version jar

Vanilla 1.20.1.jar on -cp creates automatic module _1._20._1 which
conflicts with Forge minecraft module. Use forge-*-client.jar from
libraries/ instead (contains all MC classes). Falls back to vanilla.

launcher/launcher/src/main/java/sashegdev/zernmc/launcher/minecraft/launch/LaunchCommandBuilder.java

@@ -63,14 +63,16 @@ public class LaunchCommandBuilder {
             // always use ModLauncher as main class (version.json might have
             // BootstrapLauncher which breaks with Java module system)
             if ("forge".equals(loaderType) || "neoforge".equals(loaderType)) {
-                // Forge manifest's -p is a HARDCODED list of 8 bootstrap JARs
+                // Forge: vanilla version jar on -cp creates automatic module _1._20._1
-                // (bootstraplauncher, securejarhandler, asm-*, JarJarFileSystems)
+                // which conflicts with Forge's "minecraft" module → split-package error.
-                // using ${library_directory}. Version jar is NOT on -p.
+                // Fix: use patched Forge client jar instead of vanilla version jar.
-                // Put version jar on -cp only → PreparableReloadListener loadable,
+                // The patched jar (forge-1.20.1-47.4.20-client.jar) is created by the
-                // no _1._20._1 vs minecraft split-package conflict.
+                // Forge installer in libraries/ and contains all MC classes including
+                // PreparableReloadListener. It is NOT listed in version.json manifest,
+                // so we find and add it separately.
                 String classpath;
                 if (manifest != null) {
-                    classpath = buildClasspathFromManifest(manifest, true);
+                    classpath = buildClasspathFromManifest(manifest, false);
                 } else {
                     classpath = "";
                 }
@@ -78,6 +80,21 @@ public class LaunchCommandBuilder {
                     classpath = buildVanillaClasspath();
                 }
 
+                // Find and add the patched Forge client jar
+                Path patchedJar = findPatchedForgeJar();
+                if (patchedJar != null) {
+                    String sep = System.getProperty("os.name").toLowerCase().contains("win") ? ";" : ":";
+                    classpath = patchedJar.toAbsolutePath().toString() + sep + classpath;
+                    System.out.println(ZAnsi.green("  Added patched Forge jar: " + patchedJar.getFileName()));
+                } else {
+                    System.out.println(ZAnsi.yellow("  No patched Forge client jar found, falling back to vanilla version jar"));
+                    Path versionJar = findVersionJar();
+                    if (versionJar != null && isValidJar(versionJar)) {
+                        String sep = System.getProperty("os.name").toLowerCase().contains("win") ? ";" : ":";
+                        classpath = versionJar.toAbsolutePath().toString() + sep + classpath;
+                    }
+                }
+
                 command.add("-cp");
                 String cpFile = writeClasspathFile(classpath);
                 command.add(cpFile);
@@ -640,6 +657,35 @@ public class LaunchCommandBuilder {
         return null;
     }
 
+    /**
+     * Find the patched Forge client jar in libraries/.
+     * The Forge installer creates it via binarypatcher processor:
+     * libraries/net/minecraftforge/forge/<version>/forge-<version>-client.jar
+     * This jar contains all vanilla MC classes + Forge patches.
+     * Using it instead of vanilla 1.20.1.jar avoids the _1._20._1 vs minecraft
+     * split-package conflict, because the filename produces a different automatic
+     * module name.
+     */
+    private Path findPatchedForgeJar() {
+        Path librariesDir = instance.getPath().resolve("libraries");
+        String mcVersion = instance.getMinecraftVersion();
+        String loaderVer = instance.getLoaderVersion();
+
+        String[] candidates = {
+            "net/minecraftforge/forge/" + mcVersion + "-" + loaderVer + "/forge-" + mcVersion + "-" + loaderVer + "-client.jar",
+            "net/neoforged/neoforge/" + mcVersion + "-" + loaderVer + "/neoforge-" + mcVersion + "-" + loaderVer + "-client.jar"
+        };
+
+        for (String candidate : candidates) {
+            Path jarPath = librariesDir.resolve(candidate);
+            if (Files.exists(jarPath) && isValidJar(jarPath)) {
+                return jarPath;
+            }
+        }
+
+        return null;
+    }
+
     private String mavenToPath(String mavenName) {
         String[] parts = mavenName.split(":");
         if (parts.length < 3) return mavenName;