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/api/ipot/client.rs

@@ -0,0 +1,62 @@
+use std::time::Duration;
+
+use crate::error::IdxError;
+
+const USER_AGENT: &str = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/122.0.0.0 Safari/537.36";
+/// Tries per request. In some stretches of seconds the server answers its
+/// own nginx 404 page to a third or more of requests and then recovers, so a
+/// 404 is retried (an unknown ticker is a 200, not a 404).
+const ATTEMPTS: u32 = 5;
+const FIRST_PAUSE: Duration = Duration::from_millis(250);
+
+pub(super) struct IpotClient {
+    agent: ureq::Agent,
+}
+
+impl IpotClient {
+    pub(super) fn new() -> Self {
+        let agent: ureq::Agent = ureq::Agent::config_builder()
+            .timeout_connect(Some(Duration::from_secs(5)))
+            .timeout_recv_body(Some(Duration::from_secs(10)))
+            // A server that stalls before sending headers must not hold a
+            // call (and an MCP call slot) forever.
+            .timeout_global(Some(Duration::from_secs(20)))
+            .build()
+            .into();
+        Self { agent }
+    }
+
+    pub(super) fn fetch_html(&self, url: &str) -> Result<String, IdxError> {
+        let mut pause = FIRST_PAUSE;
+        let mut last = None;
+        for attempt in 1..=ATTEMPTS {
+            if attempt > 1 {
+                std::thread::sleep(pause);
+                pause *= 2;
+            }
+            let response = self
+                .agent
+                .get(url)
+                .header("User-Agent", USER_AGENT)
+                .header("Accept", "text/html")
+                .call();
+            match response {
+                Ok(ok) => {
+                    return ok
+                        .into_body()
+                        .read_to_string()
+                        .map_err(|e| IdxError::Http(format!("ipot broker summary: {e}")));
+                }
+                Err(ureq::Error::StatusCode(code)) if code == 404 || code >= 500 => {
+                    last = Some(code);
+                }
+                Err(ureq::Error::StatusCode(429)) => return Err(IdxError::RateLimited),
+                Err(err) => return Err(IdxError::Http(format!("ipot broker summary: {err}"))),
+            }
+        }
+        Err(IdxError::Http(format!(
+            "ipot broker summary: status {} after {ATTEMPTS} attempts",
+            last.unwrap_or_default()
+        )))
+    }
+}