feat: login-free Stockbit intraday and quote provider

Read Stockbit's public symbol page (__NEXT_DATA__) with a plain ureq
request and an honest user agent: `stocks intraday` (today by the
minute, any provider), `--provider stockbit` quotes with the best
bid/offer only while the market is open, and MCP `get_intraday`.

- UPSTREAMBLOCKED error (exit 4) for a CDN 403; never retried
- requests paced 1 s apart across idx processes, 5 s snapshot cache
- global --provider flag (yahoo, msn, stockbit)
- roadmap: L1 ships from Stockbit; depth stays a login follow-on

crates/idx-core/src/config.rs

@@ -13,10 +13,14 @@ use crate::error::IdxError;
 pub const OUTPUT_VALUES: &[&str] = &["auto", "table", "json", "jsonl"];
 
 #[derive(Debug, Clone, Copy, Deserialize, PartialEq, Eq)]
+#[cfg_attr(feature = "clap", derive(clap::ValueEnum))]
 #[serde(rename_all = "lowercase")]
+#[cfg_attr(feature = "clap", value(rename_all = "lower"))]
 pub enum ProviderKind {
     Yahoo,
     Msn,
+    // Opt-in: quotes from Stockbit's public page (delayed, no login).
+    Stockbit,
 }
 
 #[derive(Debug, Clone, Copy, Deserialize, PartialEq, Eq)]
@@ -34,6 +38,7 @@ impl ProviderKind {
         match self {
             Self::Yahoo => "yahoo",
             Self::Msn => "msn",
+            Self::Stockbit => "stockbit",
         }
     }
 
@@ -42,9 +47,11 @@ impl ProviderKind {
             Ok(Self::Yahoo)
         } else if value.eq_ignore_ascii_case("msn") {
             Ok(Self::Msn)
+        } else if value.eq_ignore_ascii_case("stockbit") {
+            Ok(Self::Stockbit)
         } else {
             Err(IdxError::ConfigError(format!(
-                "invalid provider '{value}' (expected yahoo or msn)"
+                "invalid provider '{value}' (expected yahoo, msn, or stockbit)"
             )))
         }
     }