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

docs/ARCHITECTURE.md

@@ -53,8 +53,8 @@ command parses arguments, makes one idx-core call, and renders the result.
 - **Yahoo Finance** — default/auto history source. Reliable OHLCV data via `/v8/finance/chart/`.
 - **IPOT (Indo Premier)** — broker summaries, whichever provider is selected
   (`BrokerSummaryProvider`). See [Broker Summaries](#broker-summaries).
-- **IDX (idx.co.id)** — the daily trading summary behind foreign flow
-  (`TradingSummaryProvider`). See [Foreign Flow](#foreign-flow).
+- **IDX (idx.co.id)** — the daily trading summary behind `stocks flow
+  --source idx` (`TradingSummaryProvider`). See [Foreign Flow](#foreign-flow).
 
 ### MSN Endpoints
 - `https://assets.msn.com/service/`: quotes, charts, equities, earnings,
@@ -116,7 +116,13 @@ WIB as the fallback.
   never summed as zero. A change to the rules gets a new method name.
 
 ### Foreign Flow
-- `GET https://www.idx.co.id/primary/TradingSummary/GetStockSummary`
+- The default source is IPOT: per session, the all-investor broker summary
+  gives the value traded and the foreign one (`fd=F`) gives foreign buying
+  (its footer), the foreign net, and selling (buying less the net), 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. An empty all-investor
+  summary counts as no session.
+- `--source idx`: `GET https://www.idx.co.id/primary/TradingSummary/GetStockSummary`
   `?length=9999&start=0&date=YYYYMMDD` answers one JSON row per stock for the
   session: prices, volume, value, `ForeignBuy`/`ForeignSell` (shares) and
   non-regular volume and value. `Volume`, `Value` and the foreign figures are