Refuse a Gaia answer that came back short, and read the body inside the retry

The merge gate asks whether Gaia contributed any stars, never how many. The TAP service truncates
on its own timeout and still serves a well-formed CSV with a 200, ordered by magnitude — so a half
answer is the bright half, which is the half HYG overlaps. Every gate passes: Gaia stars are
present, HYG survivors go down rather than up, unmerged twins can only fall. The weekly job would
publish a catalogue missing two hundred thousand stars and the runner would cache it for the weeks
after. `fetchGaiaStars` now refuses fewer than 95% of the 412 765 rows its query holds, as its
sibling query already did, and refuses an answer that fills the row limit.

`fetchText` retried the request but not the body: a connection reset part-way through the 57 MB
CSV rejected out of the loop, with no wait and no second attempt. The read now happens inside it.

Also corrected: the merge gate's account of the HYG survivors (two thirds of them are stars Gaia
measures but the main query never downloads, since Gaia puts them past the 250 pc cutoff), and the
refresh workflow's comment on what happens when the archive is unreachable.

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

tools/etl/sources/gaia.ts

@@ -38,9 +38,29 @@ const CATALOGUE_EPOCH = 2000.0;
  * past anything this map draws, so the limit here is a payload decision: the catalogue is baked
  * into a static asset that a browser downloads before the first frame.
  */
-const DISTANCE_CUTOFF_PC = Number(process.env['ETL_GAIA_DISTANCE_PC'] ?? 250);
-const MAGNITUDE_LIMIT = Number(process.env['ETL_GAIA_MAGNITUDE_LIMIT'] ?? 12);
-const ROW_LIMIT = Number(process.env['ETL_GAIA_ROW_LIMIT'] ?? 500000);
+const DEFAULT_DISTANCE_CUTOFF_PC = 250;
+const DEFAULT_MAGNITUDE_LIMIT = 12;
+const DEFAULT_ROW_LIMIT = 500_000;
+const DISTANCE_CUTOFF_PC = Number(process.env['ETL_GAIA_DISTANCE_PC'] ?? DEFAULT_DISTANCE_CUTOFF_PC);
+const MAGNITUDE_LIMIT = Number(process.env['ETL_GAIA_MAGNITUDE_LIMIT'] ?? DEFAULT_MAGNITUDE_LIMIT);
+const ROW_LIMIT = Number(process.env['ETL_GAIA_ROW_LIMIT'] ?? DEFAULT_ROW_LIMIT);
+
+/**
+ * How many rows the scheduled job's own query holds: 412 765, and DR3 is a finished data release,
+ * so that number only moves when the query does.
+ *
+ * Checked because a short answer looks exactly like a complete one. The TAP service truncates on
+ * its own timeout and still serves a well-formed CSV with a 200, and the rows are ordered by
+ * magnitude, so what comes back is the bright half — the half HYG overlaps. The merge gate in
+ * `build.ts` would then see Gaia stars present, fewer HYG survivors and fewer unmerged twins, and
+ * pass a catalogue missing two hundred thousand stars, which the weekly job would publish and the
+ * runner would cache for the weeks after it. Same failure, and same guard, as
+ * {@link MIN_USABLE_HIP_DISTANCES} below.
+ *
+ * Only checked for that query: the environment overrides exist to fetch a smaller slice on purpose.
+ */
+const DEFAULT_QUERY_ROWS = 412_765;
+const MIN_ROW_SHARE = 0.95;
 
 /**
  * Relative parallax error above which a star is dropped: a parallax measured to worse than 20%
@@ -92,6 +112,16 @@ export async function fetchGaiaStars(): Promise<StarRecord[]> {
   // response arrived, not what it answered.
   const csv = await fetchTextCached(url, `gaia-dr3-${createHash('sha1').update(url).digest('hex').slice(0, 8)}.csv`);
   const rows = parseCsvObjects(csv);
+  const jobsQuery = DISTANCE_CUTOFF_PC === DEFAULT_DISTANCE_CUTOFF_PC && MAGNITUDE_LIMIT === DEFAULT_MAGNITUDE_LIMIT && ROW_LIMIT === DEFAULT_ROW_LIMIT;
+  if (jobsQuery && rows.length < DEFAULT_QUERY_ROWS * MIN_ROW_SHARE) {
+    throw new Error(
+      `Gaia returned ${rows.length} rows, not the ~${DEFAULT_QUERY_ROWS} this query holds — the answer was cut short, ` +
+        'or was an error page served with a 200; delete tools/etl/.cache/gaia-dr3-*.csv once the archive answers properly'
+    );
+  }
+  if (rows.length >= ROW_LIMIT) {
+    throw new Error(`Gaia returned the query's own ${ROW_LIMIT}-row limit, so it is the limit deciding what the map holds; raise ETL_GAIA_ROW_LIMIT.`);
+  }
   const stars: StarRecord[] = [];
 
   rows.forEach((row, index) => {