perf(core): cache $dynamicAnchor fallback index on context

[aqeelat]

When dynamicScope has no entry for an anchor, resolveDynamicRef scans all components.schemas on every call. For specs with many $dynamicRefs, this is O(numDynamicRefs × numSchemas).

Two improvements:

1. Cache — lazily build a Map<anchorName, string[]> on the context so the scan runs once per spec
2. Short-circuit — track first match + count instead of collecting all matches; bail early when ambiguous

Location: packages/core/src/resolvers/ref.ts around line 456-475.