fix: mark only IDX quotes behind the session; test the wiring

Review findings on #71:
- The IDX session rule flagged non-IDX data: US screen rows on Monday
  evening WIB, and JP/HK/SG rows on their own holidays. Only `.JK`
  quotes and `--region id` screens are marked now.
- Nothing tested the marking call sites. A screen test checks IDX rows
  online and offline, and US rows unmarked. A quote test reads a cache
  entry in 0.3.3's format (no field) offline.
- The get_quotes description now says the flag also means the stock did
  not trade, or an exchange holiday; screen_stocks mentions the field.
- latest_session is pub(crate).

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

docs/ARCHITECTURE.md

@@ -78,8 +78,10 @@ WIB as the fallback.
 - Quotes expose nullable `as_of` last-trade timestamps in WIB, and
   `behind_session` when `as_of` is before the latest weekday whose session has
   ended (16:15 WIB). It is computed on return, not cached; MSN can lag the close
-  by hours, and on an exchange holiday every quote reads as behind. Table output
-  notes behind quotes, and calls those over seven days old not current.
+  by hours, and on an exchange holiday every quote reads as behind. Only `.JK`
+  quotes and `--region id` screens are marked; other exchanges keep other hours.
+  Table output notes behind quotes, and calls those over seven days old not
+  current.
 - `change`, `change_pct`, and `volume` are nullable when the provider omits them;
   JSON consumers must not assume these fields are always numeric.
 - Listed MSN instruments without a price return `NOMARKETDATA`; unknown symbols