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/broker/mod.rs

@@ -114,7 +114,11 @@ impl Broker<'_> {
                 source: SOURCE,
             },
             policy,
-            || provider.broker_summary(&code, query),
+            || {
+                provider
+                    .broker_summary(&code, query)
+                    .and_then(|raw| check_echoed_dates(query, raw))
+            },
             |raw| Ok(summary_from_raw(&resolved, query, raw)),
         )
     }
@@ -207,6 +211,25 @@ pub(crate) fn summary_from_raw(
     summary
 }
 
+/// The page echoes the dates it covers; another range's figures must not be
+/// cached or reported as the requested one's.
+fn check_echoed_dates(
+    query: &BrokerQuery,
+    raw: BrokerSummaryRaw,
+) -> Result<BrokerSummaryRaw, IdxError> {
+    if raw.from == Some(query.from) && raw.to == Some(query.to) {
+        return Ok(raw);
+    }
+    let echoed = |date: Option<NaiveDate>| date.map_or("none".to_string(), |date| date.to_string());
+    Err(IdxError::ParseError(format!(
+        "ipot broker summary: asked for {} to {}, the page covers {} to {}",
+        query.from,
+        query.to,
+        echoed(raw.from),
+        echoed(raw.to)
+    )))
+}
+
 fn check_range(query: &BrokerQuery, today: NaiveDate) -> Result<(), IdxError> {
     if query.from > query.to {
         return Err(IdxError::InvalidInput(format!(
@@ -381,4 +404,29 @@ mod tests {
             );
         }
     }
+
+    #[test]
+    fn test_check_echoed_dates_rejects_another_sessions_page() {
+        let friday = date(2026, 10, 16);
+        let page = || BrokerSummaryRaw {
+            from: Some(friday),
+            to: Some(friday),
+            ..BrokerSummaryRaw::default()
+        };
+        assert!(check_echoed_dates(&query(friday, friday), page()).is_ok());
+        let thursday = date(2026, 10, 15);
+        for (asked, raw) in [
+            (query(thursday, thursday), page()),
+            (query(thursday, friday), page()),
+            (query(friday, friday), BrokerSummaryRaw::default()),
+        ] {
+            assert!(
+                matches!(
+                    check_echoed_dates(&asked, raw),
+                    Err(IdxError::ParseError(_))
+                ),
+                "{asked:?}"
+            );
+        }
+    }
 }