feat: foreign flow (idx stocks flow, get_foreign_flow)

`idx stocks flow SYMBOL` shows what foreign investors bought and sold of a
stock, session by session, from IDX's own daily trading summary: foreign
buy, sell and net volume, net value at the session's average price, and
the share of value traded outside the regular board, with range totals.
Latest 10 sessions by default (--days, at most 60) or --from/--to. The MCP
server exposes it as get_foreign_flow.

- api/idx: pure parser for GetStockSummary (rows dated another session
  are dropped) and a provider over curl-impersonate's profile fallback
  with a 30 s limit; Cloudflare passes only some fingerprints.
- flow: each session is cached whole by date, so other stocks' flows for
  the same sessions need no request. Weekends are never requested.
- Idx::idx_code is shared by broker and flow for the .JK-only check.

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

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

@@ -1,3 +1,4 @@
+pub mod idx;
 pub mod ipot;
 #[cfg(feature = "testing")]
 pub mod mock;
@@ -54,6 +55,15 @@ pub trait BrokerSummaryProvider {
     ) -> Result<ipot::BrokerSummaryRaw, IdxError>;
 }
 
+/// Every stock's trading summary for one session (IDX's own site).
+pub trait TradingSummaryProvider {
+    /// Empty for a day without trading.
+    fn stock_summary(
+        &self,
+        date: chrono::NaiveDate,
+    ) -> Result<Vec<crate::flow::types::StockDay>, IdxError>;
+}
+
 pub trait FundamentalsProvider {
     fn fundamentals(&self, symbol: &str) -> Result<Fundamentals, IdxError>;
 }
@@ -279,6 +289,14 @@ pub fn broker_provider() -> Box<dyn BrokerSummaryProvider> {
     Box::new(ipot::IpotProvider::new())
 }
 
+pub fn trading_summary_provider() -> Box<dyn TradingSummaryProvider> {
+    #[cfg(feature = "testing")]
+    if mock::enabled() {
+        return Box::new(mock::MockTradingSummaryProvider);
+    }
+    Box::new(idx::IdxSiteProvider)
+}
+
 /// Today in WIB, the date IDX trades on.
 pub(crate) fn today_wib() -> chrono::NaiveDate {
     #[cfg(feature = "testing")]