Fix 12: Add forge client jar to classpath (missing from version JSON libs)

The forge patched client jar (forge-<version>-client.jar) contains all
Minecraft classes with Automatic-Module-Name: minecraft, but it is NOT
listed in the version JSON libraries array - so buildClasspathFromManifest
does not include it on the -cp. FML discovers it by scanning the library
directory, but securejarhandler only processes classpath entries to create
named modules.

This fix explicitly finds and prepends forge-<version>-client.jar to the
classpath when launching Forge/NeoForge, so securejarhandler can create
the minecraft module from it.

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

@@ -63,12 +63,7 @@ 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)) {
-                // Build classpath from manifest libraries only.
+                // Build classpath from manifest libraries.
-                // The forge client jar (forge-*-client.jar) on the classpath
-                // contains the patched Minecraft classes and has
-                // Automatic-Module-Name: minecraft in its MANIFEST.
-                // We modify the ignoreList below to NOT exclude it,
-                // so securejarhandler creates the "minecraft" module from it.
                 String classpath;
                 if (manifest != null) {
                     classpath = buildClasspathFromManifest(manifest, false);
@@ -79,6 +74,25 @@ public class LaunchCommandBuilder {
                     classpath = buildVanillaClasspath();
                 }
 
+                // The forge client jar (forge-*-client.jar) is NOT listed in the
+                // version JSON's libraries array, so it's NOT on the classpath.
+                // FML discovers it by scanning the library directory, but
+                // securejarhandler only processes classpath entries.
+                // We add it explicitly so securejarhandler creates the "minecraft"
+                // module from its Automatic-Module-Name: minecraft.
+                String forgeVersion = instance.getMinecraftVersion() + "-" + instance.getLoaderVersion();
+                Path forgeClientJar = instance.getPath().resolve("libraries")
+                    .resolve("net/minecraftforge/forge")
+                    .resolve(forgeVersion)
+                    .resolve("forge-" + forgeVersion + "-client.jar");
+                if (Files.exists(forgeClientJar) && isValidJar(forgeClientJar)) {
+                    String sep = System.getProperty("os.name").toLowerCase().contains("win") ? ";" : ":";
+                    classpath = forgeClientJar.toAbsolutePath().toString() + sep + classpath;
+                    System.out.println(ZAnsi.green("  Added forge client jar: " + forgeClientJar.getFileName()));
+                } else {
+                    System.out.println(ZAnsi.yellow("  Forge client jar not found at " + forgeClientJar));
+                }
+
                 command.add("-cp");
                 String cpFile = writeClasspathFile(classpath);
                 command.add(cpFile);