Replace Ant build with native Gradle build for DXF library

- Removed ant.importBuild("build.xml")
- Added standalone Gradle build logic using Kotlin DSL
- Configured Java 17 toolchain
- Added core.jar presence check
- Created custom dxfJar and clean tasks

This removes reliance on Ant for the DXF library as part of the migration effort described in processing#981

Author: LalitNarayanYadav

java/libraries/dxf/build.gradle.kts

@@ -1 +1,37 @@
-ant.importBuild("build.xml")
+plugins {
+    java
+}
+
+
+java {
+    toolchain {
+        languageVersion.set(JavaLanguageVersion.of(17))
+    }
+}
+
+val coreJar = file("../../../core/library/core.jar")
+
+dependencies {
+    implementation(files(coreJar))
+}
+
+tasks.register("checkCore") {
+    doFirst {
+        if (!coreJar.exists()) {
+            throw GradleException("Missing core.jar at $coreJar. Please build the core module first.")
+        }
+    }
+}
+
+tasks.register<Jar>("dxfJar") {
+    dependsOn("checkCore", "classes")
+    archiveBaseName.set("dxf")
+    destinationDirectory.set(file("library"))
+    from(sourceSets.main.get().output)
+}
+
+tasks.register("clean") {
+    doLast {
+        delete("build", "library/dxf.jar")
+    }
+}