Nested XR identity not preserved during diff

[jcogilvie]

Expected behavior:

• Nested XRs should be matched to their existing cluster instances by crossplane.io/composition-resource-name
  annotation
• Existing nested XR names and crossplane.io/composite labels should be preserved
• Managed resources owned by nested XRs should correctly show as unchanged (or show actual modifications if
  changed)

Actual behavior:

• Nested XRs are treated as new resources with generated names
• All managed resources owned by nested XRs appear with "---" (removed) and "+++" (added) markers
• The diff shows dozens/hundreds of resources being replaced when they should be unchanged
• Nested XR names display with "(generated)" suffix instead of their actual cluster names

How can we reproduce it?

1. Create a composition that produces nested XRs (an XR that creates other XRs as composed resources)
2. Apply an XR or Claim using this composition to a cluster with Crossplane installed
3. Wait for all resources to become available
4. Modify the parent XR/Claim in any way (e.g., add labels, change a field value)
5. Run crossplane-diff xr modified-xr.yaml

Observed: Diff shows all nested managed resources as removed/re-added
Expected: Diff correctly identifies nested XRs and shows only actual changes