can ytt auto-explain why overlay matching did not match expected number of nodes

**Describe the problem/challenge you have**
Users may run into a challenge determining why their overlay does not match expected number of nodes. (e.g. if `overlay.subset({"apiVersion": "cert-manager.io/v1"})` but the actual node has `cert-manager.io/v1alpha1`). it would be interesting to somehow let the user know potential nodes that were "almost" matched.

no clue how to implement this.

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The issue does not name any implementation files, tests, or entry points. Start by tracing ytt's overlay matching behavior and how it reports mismatched node counts; done would mean a documented or tested way for users to understand near-matches without changing the requested matching semantics.

Written by the indexing model from the issue text.