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

crates/idx-mcp/tests/golden/tools.json

@@ -397,7 +397,7 @@
     "name": "get_price_history"
   },
   {
-    "description": "Latest quotes (price, change, volume, 52-week range, market cap) for one or more IDX tickers. `as_of` is the last trade in WIB; `behind_session` is true when that is before the latest IDX session, so the price is not that session's close.",
+    "description": "Latest quotes (price, change, volume, 52-week range, market cap) for one or more IDX tickers. `as_of` is the last trade in WIB; `behind_session` is true when that is before the latest IDX session, so the price is not that session's close: the provider lags, the stock did not trade, or the day was an exchange holiday.",
     "inputSchema": {
       "$schema": "https://json-schema.org/draft/2020-12/schema",
       "properties": {
@@ -457,7 +457,7 @@
     "name": "ownership_sync"
   },
   {
-    "description": "Run a stock screen (top/worst performers, high dividend, low P/E, 52-week highs/lows, high volume, large cap).",
+    "description": "Run a stock screen (top/worst performers, high dividend, low P/E, 52-week highs/lows, high volume, large cap). Rows carry `as_of` and, for the IDX region, `behind_session` as in get_quotes.",
     "inputSchema": {
       "$schema": "https://json-schema.org/draft/2020-12/schema",
       "properties": {