bug: xref-db caches empty array for missing results, preventing future resolution

[marcoscaceres]

Description

In src/core/xref-db.js:90, cacheXrefData() stores results.get(query.id) ?? [] into IDB. When the xref API returns no result for a query ID, an empty array is cached.

On subsequent page loads, resolveXrefCache() finds a cache hit (the empty array) and considers the query resolved, never re-fetching from the network. This permanently suppresses xref results for terms that were temporarily missing from the API.

Steps to reproduce

1. Author uses [=some-term=] that is temporarily not in the xref database
2. ReSpec fetches xref API, gets no result
3. Empty array [] is cached in IDB for that query
4. Term is later added to the xref database
5. ReSpec never re-fetches because IDB has a cache hit

Suggested fix

Skip caching queries with no results:

for (const query of queries) {
  const result = results.get(query.id);
  if (result?.length) {
    tx.objectStore(STORE_NAME).add({ query, result });
  }
}

Found by

Adversarial gate holdout test (blind review of PR #5169).