Say so when Gaia is missing, rather than as 68 000 unmatched stars

With the merge gate in place, a Gaia DR3 outage no longer ships a HYG-only
catalogue: the ETL skips the unreachable source, and validateMerge then fails
on the survivor count. That is the right outcome and the wrong message: "68 000
HYG stars found no Gaia counterpart" sends the reader looking at the merge.
Gaia contributing nothing is now checked first, by name.

Two comments said Gaia was best-effort, in data-refresh.yml and on the merge
in fetchStars. They now say what happens instead.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016jxMkwA2rbicdGxHosecYi

tools/etl/fetchStars.ts

@@ -119,9 +119,9 @@ export async function fetchStars(): Promise<StarRecord[]> {
 /**
  * Unions HYG with every other positional source that is wired in and reachable.
  *
- * A source that cannot be reached is reported and skipped rather than failing the run. That is
- * not defensive padding: the archives this would draw on are frequently unavailable, and a build
- * that produces a smaller catalogue is far better than one that produces none.
+ * A source that cannot be reached is reported and skipped here rather than thrown, so a run still
+ * gets as far as validation and says what it has. Whether that may be published is decided
+ * there: `validateMerge` in build.ts refuses a catalogue Gaia contributed nothing to.
  */
 async function mergeWithOtherSources(hygStars: StarRecord[]): Promise<StarRecord[]> {
   const others = positionalSources().filter((source) => source.id !== 'hyg');