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-mcp/tests/golden/tools.json

@@ -236,7 +236,7 @@
     "name": "get_company"
   },
   {
-    "description": "Foreign investors' buying and selling of one IDX ticker, session by session (oldest first), from IDX's daily trading summary: foreign buy, sell and net volume in shares, `foreign_net_value` (net shares at the session's average price, an estimate, IDR), `change_bps`, and `non_regular_bps` (share of all value traded outside the regular board). Volume, value and foreign figures are the regular board. Totals cover the whole range; `foreign_share_bps` is foreign investors' share of regular-board trading. End of day.",
+    "description": "Foreign investors' buying and selling of one IDX ticker, session by session (oldest first), with range totals; end of day. With `source` \"ipot\" (default): `foreign_buy_value`, `foreign_sell_value` and `foreign_net_value` in IDR across all boards, `value` traded, and `foreign_share_bps` (foreign share of trading); share counts, close and `non_regular_bps` are null, and `rounded` = true means one decimal of precision. With \"idx\": exact foreign share counts on the regular board, values estimated at the session's average price, the close, `change_bps` and `non_regular_bps`.",
     "inputSchema": {
       "$schema": "https://json-schema.org/draft/2020-12/schema",
       "properties": {
@@ -258,6 +258,15 @@
             "null"
           ]
         },
+        "source": {
+          "default": null,
+          "description": "Where the figures come from (default \"ipot\"): \"ipot\" gives foreign buy, sell and net\nvalue across all boards; \"idx\" gives exact share counts, the close and the non-regular\nshare, but needs curl-impersonate and may be blocked (HTTP 403).",
+          "enum": [
+            "ipot",
+            "idx"
+          ],
+          "type": "string"
+        },
         "symbol": {
           "description": "IDX ticker, e.g. \"BBCA\".",
           "type": "string"