Clear remaining ignored SpotBugs findings

[e6qu]

Summary

• remove the SpotBugs exclude filter so high-confidence findings are no longer ignored
• make ThemeCache color arrays private behind typed accessors
• fix the previously hidden vendored NFS/RPC and sun.net.www SpotBugs findings
• update BUGS/PLAN and SpotBugs workflow docs to reflect the unfiltered setup

Validation

• git diff --check
• ./gradlew spotbugsMain spotbugsTest --stacktrace
• ./gradlew cleanTest test --stacktrace
• ./gradlew check --stacktrace
• ./gradlew :barebones-protocol-nfs:compileJava :barebones-protocol-nfs:spotbugsMain --stacktrace