Add Gradle performance optimizations

Enable parallel builds, build caching, and increase heap size for better build performance.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: nakamura-to

gradle.properties

@@ -1,3 +1,6 @@
 group=org.domaframework.doma
 version=4.0.1-SNAPSHOT
 release.useAutomaticVersion=true
+org.gradle.parallel=true
+org.gradle.caching=true
+org.gradle.jvmargs=-Xmx2048M