Add cap4k validator addon

[LDmoxeii]

Summary

• Adds the only-engine-validator cap4k addon provider for generating checked-in Bean Validation annotation/validator skeletons.
• Hardens validator manifest validation for value types, annotation parameter types, reserved names, duplicate parameters, and scalar default literals.
• Documents same-cycle verification against a cap4k composite build.

Test Plan

• ./gradlew.bat --no-daemon "-PonlyEngine.cap4kCompositePath=C:/Users/LD_moxeii/Documents/code/only-workspace/cap4k/.worktrees/issue-84-generator-boundary" :engine-cap4k-addon:test
• ./gradlew.bat --no-daemon test attempted without composite path; blocked at dependency resolution because com.only4:cap4k-plugin-pipeline-api:0.5.0-SNAPSHOT is not available from configured repositories.

Dependency

• Depends on cap4k PR Implement generator input boundary realignment cap4k#89 for the same-cycle cap4k plugin pipeline API/SPI changes.