feat(ctrader-openapi): scaffold crate with config and error types

New workspace crate for cTrader Open API client (Protobuf over TCP+TLS).
Includes CTraderConfig, CTraderEnvironment, CTraderError, and RateLimitBucket types.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ctrader-openapi/src/error.rs

@@ -0,0 +1,84 @@
+//! cTrader Open API error types.
+
+use std::fmt;
+
+/// Errors from the cTrader Open API client.
+#[derive(Debug, thiserror::Error)]
+pub enum CTraderError {
+    /// TCP or TLS connection failed.
+    #[error("connection failed: {0}")]
+    ConnectionFailed(String),
+
+    /// TLS handshake error.
+    #[error("TLS error: {0}")]
+    TlsError(String),
+
+    /// OAuth2 token exchange failed.
+    #[error("OAuth error: {0}")]
+    OAuthError(String),
+
+    /// Application-level authentication rejected.
+    #[error("application auth failed: {0}")]
+    AppAuthFailed(String),
+
+    /// Account-level authentication rejected.
+    #[error("account auth failed: {0}")]
+    AccountAuthFailed(String),
+
+    /// Protobuf encode/decode error.
+    #[error("protocol error: {0}")]
+    ProtocolError(String),
+
+    /// Codec framing error.
+    #[error("codec error: {0}")]
+    CodecError(String),
+
+    /// Rate limit exceeded (50 req/s or 5 req/s for historical).
+    #[error("rate limit exceeded for {bucket}")]
+    RateLimitExceeded {
+        /// Which bucket was exhausted.
+        bucket: RateLimitBucket,
+    },
+
+    /// Request timed out waiting for a response.
+    #[error("request timed out after {0:?}")]
+    Timeout(std::time::Duration),
+
+    /// Server returned an error payload.
+    #[error("server error {code}: {description}")]
+    ServerError {
+        /// cTrader error code.
+        code: String,
+        /// Human-readable description.
+        description: String,
+    },
+
+    /// Client is not connected.
+    #[error("not connected")]
+    NotConnected,
+
+    /// Symbol not found in the cached symbol list.
+    #[error("unknown symbol: {0}")]
+    UnknownSymbol(String),
+}
+
+/// Rate-limit bucket categories.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum RateLimitBucket {
+    /// Standard messages: 50 per second.
+    NonHistorical,
+    /// Historical data requests: 5 per second.
+    Historical,
+}
+
+impl fmt::Display for RateLimitBucket {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::NonHistorical => f.write_str("non-historical (50/s)"),
+            Self::Historical => f.write_str("historical (5/s)"),
+        }
+    }
+}
+
+/// Convenience alias used throughout the crate.
+pub type Result<T> = std::result::Result<T, CTraderError>;