Unreferenced nodes declared as prefixItems alternatives are never reported

## Bug Report

`pattern-nodes-must-be-referenced` warns when a pattern declares a node that no relationship and no decision names. Such a node can never appear in an architecture, so it is either unfinished work or a leftover.

Its selector reads bare `prefixItems` entries only:

```
$.properties.nodes.prefixItems[*].properties.unique-id.const
```

A node declared inside `prefixItems[i].oneOf` or `prefixItems[i].anyOf` has its `unique-id` one level deeper, so the selector never reaches it. The rule cannot warn about an alternative, whatever its state.

### Steps to Reproduce:
1. Write a pattern with `webapp` and `database` connected, plus one `prefixItems` entry holding the alternatives `redis` and `memcached`.
2. Do not reference either alternative from any relationship or decision.
3. Run `calm validate`.

### Expected Result:
A warning for `redis` and one for `memcached`, as the rule already gives for an unreferenced bare node.

### Actual Result:
Nothing is reported.

| Declaration | Warns today |
|---|---|
| a bare `prefixItems` entry | yes |
| an alternative in `prefixItems[i].oneOf` or `.anyOf` | no |

### Note
A decision references its alternatives through `choices[].nodes`, which sits under `relationship-type`, and the rule's function reads every string under `relationship-type`. So an alternative that a decision offers is already counted as referenced. Widening the selector therefore warns only about an alternative that nothing offers and nothing connects.

### Environment:
`shared` (`@finos/calm-shared`), `src/spectral/rules-pattern.ts`. Reproduced on `main` at `0034591f`.

### Additional Context:
Found while scoping #2859. It is a defect in the `prefixItems` model and needs no `items` catalog to reproduce, so it is filed and fixed separately from that work.

The fix is `warn` severity and can add a warning to a pattern that is clean today. No pattern in this repository is affected.