fix(engine): keep ignored keys out of managed leaves

[taigrr]

Summary

• filter managed leaf values by ignore marker names even when no ignored value is preserved
• add a regression test covering marker-only ignored keys passed through leaf generation
• reuse the ignored-name lookup inside leaf writes

Verification

• go generate ./...
• go test -race ./internal/engine
• go test -race ./...
• go vet ./...
• go build ./...
• gofmt -l .
• staticcheck ./...