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/error.rs

@@ -34,6 +34,10 @@ pub enum IdxError {
     InvalidInput(String),
     #[error("no market data: {0}")]
     NoMarketData(String),
+    /// The upstream refused this client outright (e.g. a CDN's HTTP 403
+    /// block page). Retrying does not help; the message says what might.
+    #[error("upstream blocked: {0}")]
+    UpstreamBlocked(String),
 }
 
 #[derive(Debug, Clone, Copy, Serialize, PartialEq, Eq)]
@@ -53,6 +57,7 @@ pub enum ErrorCode {
     PdfParseError,
     InvalidInput,
     NoMarketData,
+    UpstreamBlocked,
 }
 
 /// The stable wire form frontends emit, e.g. `SYMBOLNOTFOUND`.
@@ -80,6 +85,7 @@ impl IdxError {
             Self::PdfParseError(_) => ErrorCode::PdfParseError,
             Self::InvalidInput(_) => ErrorCode::InvalidInput,
             Self::NoMarketData(_) => ErrorCode::NoMarketData,
+            Self::UpstreamBlocked(_) => ErrorCode::UpstreamBlocked,
         }
     }
 
@@ -93,7 +99,8 @@ impl IdxError {
             | ErrorCode::RateLimited
             | ErrorCode::ProviderUnavailable
             | ErrorCode::Offline
-            | ErrorCode::AuthError => 4,
+            | ErrorCode::AuthError
+            | ErrorCode::UpstreamBlocked => 4,
             ErrorCode::Unsupported => 5,
             ErrorCode::Io
             | ErrorCode::ParseError
@@ -119,4 +126,11 @@ mod tests {
         let unsupported = IdxError::Unsupported("history unavailable".to_string());
         assert_eq!(unsupported.code(), ErrorCode::Unsupported);
     }
+
+    #[test]
+    fn upstream_blocked_has_its_own_wire_code_and_network_exit() {
+        let blocked = IdxError::UpstreamBlocked("stockbit.com: HTTP 403".to_string());
+        assert_eq!(blocked.code().to_string(), "UPSTREAMBLOCKED");
+        assert_eq!(blocked.exit_code(), 4);
+    }
 }