refactor(core): move stocks fetching, caching, and analysis into idx-core

Phase 3b of docs/DESIGN.md. `stocks` handlers are now argument parsing,
one idx-core call, and rendering.

- Idx (client.rs) is the entry point: Idx::new(CoreConfig) with
  market(), financial(), and analysis(). Live lookups return
  Fetched<T> { data, source, fetched_at, stale } and take a CachePolicy
  (Default | Offline | Refresh, from the --offline/--no-cache flags).
- One cached_fetch replaces the CLI's three cache flows; they differed
  only in whether a fresh entry is read via get or get_offline when
  offline, which yields the same data and diagnostics. Report-building
  errors (technical) still skip the stale-cache fallback.
- market.rs: quotes, history (history_source is resolved first so the
  CLI's fallback note keeps its position), and a typed screener
  (ScreenFilter/ScreenRegion with the same error messages).
- financial.rs: profile, statements, earnings, sentiment, insights,
  news, with StatementFilter/EarningsFilter.
- analysis: TechnicalReport moves to analysis::report; Analysis offers
  technical, growth, valuation, risk, fundamental, and compare.
- CoreConfig::load() owns provider/exchange/TTL settings from
  config.toml and IDX_*; the CLI's IdxConfig adds output and color.
  With two different invalid settings at once, the reported one may
  differ from before; every single-error message is unchanged.
- Cache gains fresh/any/offline accessors returning fetch metadata.

Unit tests moved with their code (core 157 -> 168, CLI 17 -> 7). All
143 goldens unchanged; live A/B against main: 16/16 stocks identical.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/idx-core/src/api/mod.rs

@@ -245,6 +245,18 @@ fn msn_capability_error(subject: &str) -> IdxError {
     IdxError::Unsupported(format!("{subject}: command requires --provider msn"))
 }
 
+/// The provider that serves history for `provider` under `history_mode`.
+pub fn history_source(provider: ProviderKind, history_mode: HistoryProviderKind) -> ProviderKind {
+    match history_mode {
+        HistoryProviderKind::Yahoo => ProviderKind::Yahoo,
+        HistoryProviderKind::Msn => ProviderKind::Msn,
+        HistoryProviderKind::Auto => match provider {
+            ProviderKind::Yahoo => ProviderKind::Yahoo,
+            ProviderKind::Msn => ProviderKind::Yahoo,
+        },
+    }
+}
+
 /// Resolves a history provider based on the selected market data provider and
 /// history provider strategy.
 ///
@@ -255,14 +267,7 @@ pub fn history_provider(
     history_mode: HistoryProviderKind,
     verbose: bool,
 ) -> Result<(ProviderKind, Box<dyn HistoryProvider>), IdxError> {
-    let resolved = match history_mode {
-        HistoryProviderKind::Yahoo => ProviderKind::Yahoo,
-        HistoryProviderKind::Msn => ProviderKind::Msn,
-        HistoryProviderKind::Auto => match provider {
-            ProviderKind::Yahoo => ProviderKind::Yahoo,
-            ProviderKind::Msn => ProviderKind::Yahoo,
-        },
-    };
+    let resolved = history_source(provider, history_mode);
 
     if mock_enabled()
         && let Some(mock) = mock_history(resolved, verbose)