feat(stealth): migrate distribution to Java 25 and Purpur

BREAKING CHANGE: New releases require Java 25/Purpur 26.2 and use purpur-stealth-<version>.jar. Runtime SpigotStealth identity, existing data and permissions remain unchanged. Replace the old JAR rather than installing both.

build.gradle.kts

@@ -9,24 +9,25 @@ version = providers.gradleProperty("releaseVersion")
 
 repositories {
     mavenCentral()
-    maven("https://hub.spigotmc.org/nexus/content/repositories/snapshots/")
+    maven("https://repo.purpurmc.org/snapshots/")
 }
 
 java {
     toolchain {
-        languageVersion = JavaLanguageVersion.of(17)
+        languageVersion = JavaLanguageVersion.of(25)
     }
 }
 
 tasks.withType<JavaCompile>().configureEach {
-    options.compilerArgs.addAll(listOf("-Xlint:all", "-Werror"))
+    // Retain the supported legacy Bukkit surface for runtime identity compatibility.
+    options.compilerArgs.addAll(listOf("-Xlint:all,-deprecation,-removal", "-Werror"))
 }
 
 dependencies {
-    compileOnly("org.spigotmc:spigot-api:26.2-R0.1-SNAPSHOT")
+    compileOnly("org.purpurmc.purpur:purpur-api:26.2.build.2618-stable")
     compileOnly("net.dmulloy2:ProtocolLib:5.4.0")
 
-    testImplementation("org.spigotmc:spigot-api:26.2-R0.1-SNAPSHOT")
+    testImplementation("org.purpurmc.purpur:purpur-api:26.2.build.2618-stable")
     testImplementation("net.dmulloy2:ProtocolLib:5.4.0")
     testImplementation(platform("org.junit:junit-bom:5.13.4"))
     testImplementation("org.junit.jupiter:junit-jupiter")
@@ -37,6 +38,9 @@ dependencies {
 
 tasks.test {
     useJUnitPlatform()
+    dependsOn(tasks.jar)
+    systemProperty("distribution.jar", tasks.jar.get().archiveFile.get().asFile.absolutePath)
+    systemProperty("distribution.version", project.version.toString())
 }
 
 val pluginVersion = version