Add zvec-java bindings

[myaniu]

Summary

• Adds the java/zvec-java Maven reactor with API, JNI, and FFM backend modules.
• Adds shared backend test coverage through the API test-jar, with thin JNI/FFM subclasses and backend-specific native loader tests.
• Adds platform-aware native build and packaging support for macOS, Linux, and Windows native resources.
• Adds a JNI quickstart example and documents the multi-platform jar release flow.

Notes

• API and JNI target Java 11.
• FFM targets Java 25.
• Native libraries and packaged jars are not committed to git; multi-platform jars are intended to be published as GitHub Release assets or through the project release flow.

Test Plan

• JAVA_HOME="$HOME/.sdkman/candidates/java/25.0.2-oracle" PATH="$JAVA_HOME/bin:$PATH" mvn -q test in java/zvec-java
• JAVA_HOME="$HOME/.sdkman/candidates/java/11.0.26-amzn" PATH="$JAVA_HOME/bin:$PATH" mvn -q -pl zvec-java-jni -am install -DskipTests in java/zvec-java
• JAVA_HOME="$HOME/.sdkman/candidates/java/11.0.26-amzn" PATH="$JAVA_HOME/bin:$PATH" mvn -q compile exec:java in java/zvec-java/examples/quickstart-jni

[CLAassistant]

All committers have signed the CLA.