feat(heights): adopt Purpur distribution naming

BREAKING CHANGE: New release assets use purpur-heights-<version>.jar. Runtime SpigotHeights identity, saved player data, permissions and potion keys remain unchanged. Replace the old JAR instead of loading both distributions.

build.gradle.kts

@@ -34,6 +34,9 @@ dependencies {
 
 tasks.test {
     useJUnitPlatform()
+    dependsOn(tasks.jar)
+    systemProperty("distribution.jar", tasks.jar.get().archiveFile.get().asFile.absolutePath)
+    systemProperty("distribution.version", project.version.toString())
 }
 
 // Exercise the declared runtime's native brewing engine, not a recipe-registry double.