feat(base): adopt Purpur distribution naming

BREAKING CHANGE: New release assets use purpur-base-<version>.jar. Runtime SpigotBase identity, data folders, permissions and persistent item keys remain unchanged. Replace the old JAR rather than loading both distributions.

build.gradle.kts

@@ -38,6 +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())
 }
 
 // Verify client-only label packets against the declared runtime, without starting a server.