fix: include resource identity in single-change dyff output

[sean0407]

Summary

When dyff compares one changed resource, its output omits the Kubernetes root description. Print the existing report key before the dyff details when there is exactly one entry.

Fixes #728.

Evidence

• Before: An isolated probe against the unchanged base printed spec.parallelism and its old/new values without the resource identity.
• After: The regression test confirms the output includes default, heat-trusts, Job (batch) and the field diff.
• Validation: go test ./diff -count=1 passed.

Merge Danger

Door: Two-way

Blast Radius: Output formatting for single-entry dyff reports.