fix: increase Gradle heap to 6GB for Kotlin/Native compilation

The Kotlin/Native linker was running out of memory with 4GB on CI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Claude

gradle.properties

@@ -1,6 +1,6 @@
 org.gradle.parallel=true
 org.gradle.daemon=true
-org.gradle.jvmargs=-Xmx4g -XX:+HeapDumpOnOutOfMemoryError -Dfile.encoding=UTF-8
+org.gradle.jvmargs=-Xmx6g -XX:+HeapDumpOnOutOfMemoryError -Dfile.encoding=UTF-8
 org.gradle.caching=true
 org.gradle.configureondemand=false