[jensens]

Closes #46. Part of the 1.0 API hardening (#106 (comment)).

Problem

A StatefulSet's volumeClaimTemplates are immutable. An edit on a live VinylCache passed admission. Every following reconcile then failed on the StatefulSet update, and the cache stayed in an error loop until the edit was reverted.

Change

• ValidateUpdate passes the old object to the new ValidateVinylCacheUpdate. That function runs the existing common checks and rejects any change to spec.volumeClaimTemplates.
• The comparison uses equality.Semantic.DeepEqual, so 1Gi against 1024Mi, or nil against [], is not a change.
• A CEL transition rule was the alternative. I did not use it, because it compares quantities as strings and would reject a mere change of notation.

The error message differs from the one proposed in the issue. The issue suggested "delete and recreate the VinylCache (PVCs are preserved)". But the operator sets no persistentVolumeClaimRetentionPolicy, so the default Retain applies, and a recreated cache binds the old PVCs again at their old size. The message and the SSD how-to therefore say what actually works:

• resize the PVCs directly, if the StorageClass allows expansion,
• or delete the VinylCache and its PVCs and create it again.

Verification

• Unit table:
  • Admitted: unchanged, same size in another notation, none before and after.
  • Rejected: size changed, template added, template removed, template renamed.
  • The common checks still run on update.
• envtest against a real API server: an update that leaves the templates alone is admitted; a size change is rejected by the webhook. With the old wiring (ValidateVinylCache(newObj)) this spec fails, which shows it really exercises the old object.
• make lint, make test, make test-int are green.

Assisted-by: Claude Opus 5.5