fix(core): reject broker and trading pages that would misreport a session

From the 0.4.0 release review (Codex):
- IPOT pages with a short row, a rank out of order or a broker after a
  blank row are parse errors; skipping a row shifted later brokers up a
  rank and could flip a bandar reading (-2425 bps to -66 bps).
- A tbody end before its start is a parse error, not a panic.
- An IPOT page that echoes other dates than requested is rejected before
  it is cached, so one session's figures never pass as another's.
- An IDX trading summary with rows for other days only is an error, not
  an empty session cached for 30 days.
- Empty sessions count toward a flow's fetched_at and stale.
- The bandar imbalance uses checked i128 arithmetic: past i128 there is
  no reading instead of a wrapped one.
- The bandar table prints "No trades." only when no broker traded.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V3syn2fN2LNo5pvTSqc6eh

crates/idx-core/src/api/idx/parse.rs

@@ -38,11 +38,13 @@ fn whole(value: Option<f64>) -> Option<i64> {
 
 /// The rows of one session. A day without trading (weekend, holiday) is
 /// empty; rows dated other than `date` are dropped, so another session's
-/// figures never pass as this one's.
+/// figures never pass as this one's. Rows for other days only are an error,
+/// not an empty session, so they are never cached as one.
 pub fn parse_stock_summary(raw: &str, date: NaiveDate) -> Result<Vec<StockDay>, IdxError> {
     let response: StockSummaryResponse = serde_json::from_str(raw)
         .map_err(|e| IdxError::ParseError(format!("idx stock summary: {e}")))?;
-    Ok(response
+    let answered = response.data.len();
+    let rows: Vec<StockDay> = response
         .data
         .into_iter()
         .filter(|row| {
@@ -66,7 +68,13 @@ pub fn parse_stock_summary(raw: &str, date: NaiveDate) -> Result<Vec<StockDay>,
             non_regular_volume: whole(row.non_regular_volume),
             non_regular_value: whole(row.non_regular_value),
         })
-        .collect())
+        .collect();
+    if rows.is_empty() && answered > 0 {
+        return Err(IdxError::ParseError(format!(
+            "idx stock summary: asked for {date}, got {answered} rows for other days"
+        )));
+    }
+    Ok(rows)
 }
 
 #[cfg(test)]
@@ -101,11 +109,10 @@ mod tests {
                 .unwrap()
                 .is_empty()
         );
-        assert!(
-            parse_stock_summary(DAY, date(2026, 10, 15))
-                .unwrap()
-                .is_empty()
-        );
+        assert!(matches!(
+            parse_stock_summary(DAY, date(2026, 10, 15)),
+            Err(IdxError::ParseError(_))
+        ));
         assert!(matches!(
             parse_stock_summary("<html>Just a moment...</html>", date(2026, 10, 16)),
             Err(IdxError::ParseError(_))