feat: foreign flow from IPOT by default (--source ipot|idx)

`idx stocks flow` now reads IPOT by default, which needs nothing installed:
per session, the all-investor broker summary gives the value traded and
the foreign one gives foreign buying (its footer), the foreign net and
selling (buying less the net), across all boards, through the broker
summary cache. IPOT's foreign net matched IDX's estimate within 0.3 B on
BBCA 2026-10-12 to 16.

`--source idx` (and `source` on get_foreign_flow) keeps IDX's trading
summary: exact share counts, close and the non-regular share, behind
curl-impersonate and Cloudflare.

- ForeignFlowDay gains foreign_buy_value, foreign_sell_value and
  foreign_share_bps for both sources (estimated at the session average
  for IDX); ForeignFlow gains source, the value totals and rounded.
- The table lays out each source's figures; the live smoke runs both.

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

@@ -167,7 +167,11 @@ impl Broker<'_> {
 /// trades the lists show. `F. NVal`, the foreign net, ignores both filters:
 /// on all boards it gives the side's selling (foreign buying less the foreign
 /// net; domestic buying plus it), and with a board filter it is dropped.
-fn summary_from_raw(symbol: &str, query: &BrokerQuery, raw: BrokerSummaryRaw) -> BrokerSummary {
+pub(crate) fn summary_from_raw(
+    symbol: &str,
+    query: &BrokerQuery,
+    raw: BrokerSummaryRaw,
+) -> BrokerSummary {
     let one_side = query.investor != BrokerInvestor::All;
     let all_boards = query.board == BrokerBoard::All;
     let foreign_net = raw.foreign_net_value.filter(|_| all_boards);