feat: migrate cap4k addon to pipeline extension

[LDmoxeii]

Summary

• register a single OnlyEnginePipelineExtensionProvider through the new cap4k Pipeline Extension ServiceLoader SPI
• expose the existing enum translation and validator artifact addons as declarative extension contributions
• migrate README configuration from cap4kAddon(...) to cap4kPipelineExtension(...) and contribution-scoped options
• replace the legacy per-addon service descriptor and consolidate ServiceLoader coverage at the extension root
• update Composite Build CI to follow cap4k master instead of a deleted historical branch

Dependency

• feat: add managed field policies and pipeline extensions cap4k#150 has been merged. The required Pipeline Extension API is now available on cap4k master.

Verification

• GitHub CI Composite Build passed against cap4k master: ./gradlew check -PonlyEngine.cap4kCompositePath="$GITHUB_WORKSPACE/cap4k"
• Pipeline Extension ServiceLoader smoke passed
• direct JUnit Platform execution passed: 23/23 tests
• git diff --check passed
• CI run: https://github.com/LDmoxeii/only-engine/actions/runs/30679003683

[LDmoxeii]

CI dependency note: the workflow now correctly checks cap4k master instead of the deleted historical branch. The current failure is expected while LDmoxeii/cap4k#150 is unmerged: PipelineExtensionProvider, PipelineExtensionDescriptor, and related API symbols are not yet present on cap4k master. After #150 merges, rerun this check before marking the PR ready.