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-core/src/api/types.rs

@@ -51,8 +51,9 @@ pub struct Quote {
     #[serde(default)]
     pub as_of: Option<DateTime<FixedOffset>>,
     /// True when `as_of` is before the latest IDX session that has ended: the
-    /// provider has not caught up yet (MSN can lag hours after the close), or
-    /// the stock did not trade. False when `as_of` is unknown. Set when the
+    /// provider has not caught up yet (MSN can lag hours after the close), the
+    /// stock did not trade, or the day was an exchange holiday. False when
+    /// `as_of` is unknown and for non-IDX symbols and regions. Set when the
     /// quote is returned, not when it is cached.
     #[serde(default)]
     pub behind_session: bool,