feat(core): broker summaries from IPOT

Adds a BrokerSummaryProvider capability served by Indo Premier's public
broker summary page: the top 10 buyers and sellers of a stock by lots over
a date range, with totals. No login or key.

- api/ipot: pure parser (decimal scaling of `1.6 M`-style figures), a
  client that retries the server's spurious 404s, and a fixture mock.
- broker: Broker::summary/latest over the cache (7 days for closed ranges,
  5 minutes for today), range checks, and IDX member names bundled from
  GetBrokerCodeList.
- foreign_net_value is null for filtered summaries: the source does not
  apply the filters to it.

The endpoint's behavior was first mapped by fzn0x/idx (MIT).

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

crates/idx-core/src/client.rs

@@ -86,6 +86,10 @@ impl Idx {
         crate::market::Market { idx: self }
     }
 
+    pub fn broker(&self) -> crate::broker::Broker<'_> {
+        crate::broker::Broker { idx: self }
+    }
+
     pub fn financial(&self) -> crate::financial::Financial<'_> {
         crate::financial::Financial { idx: self }
     }