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

.github/workflows/data-refresh.yml

@@ -41,7 +41,8 @@ jobs:
 
       # The ETL's cache directory is gitignored and this is a fresh runner, so every source is
       # fetched live (~50-100 MB). A failed fetch fails the run by design — no refresh is
-      # better than a partial one — except Gaia, which the ETL itself treats as best-effort.
+      # better than a partial one. That includes Gaia: the ETL skips it when unreachable, and
+      # the merge gate in build.ts then refuses a catalogue it contributed nothing to.
       - name: Rebuild the datasets
         run: npm run etl