chore: rename quicproquo → quicprochat in Rust workspace

Rename all crate directories, package names, binary names, proto
package/module paths, ALPN strings, env var prefixes, config filenames,
mDNS service names, and plugin ABI symbols from quicproquo/qpq to
quicprochat/qpc.

crates/quicprochat-rpc/src/error.rs

@@ -0,0 +1,74 @@
+//! RPC error types.
+
+/// Status codes for RPC responses.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(u8)]
+pub enum RpcStatus {
+    /// Request succeeded.
+    Ok = 0,
+    /// Client sent a malformed request.
+    BadRequest = 1,
+    /// Authentication required or token invalid.
+    Unauthorized = 2,
+    /// Caller lacks permission for this operation.
+    Forbidden = 3,
+    /// Requested resource not found.
+    NotFound = 4,
+    /// Rate limit exceeded.
+    RateLimited = 5,
+    /// Request deadline exceeded (server-side timeout).
+    DeadlineExceeded = 8,
+    /// Server is shutting down (draining).
+    Unavailable = 9,
+    /// Internal server error.
+    Internal = 10,
+    /// Method not recognized.
+    UnknownMethod = 11,
+}
+
+impl RpcStatus {
+    /// Decode a status byte. Returns `None` for unknown values.
+    pub fn from_u8(v: u8) -> Option<Self> {
+        match v {
+            0 => Some(Self::Ok),
+            1 => Some(Self::BadRequest),
+            2 => Some(Self::Unauthorized),
+            3 => Some(Self::Forbidden),
+            4 => Some(Self::NotFound),
+            5 => Some(Self::RateLimited),
+            8 => Some(Self::DeadlineExceeded),
+            9 => Some(Self::Unavailable),
+            10 => Some(Self::Internal),
+            11 => Some(Self::UnknownMethod),
+            _ => None,
+        }
+    }
+}
+
+/// Errors that can occur in the RPC layer.
+#[derive(Debug, thiserror::Error)]
+pub enum RpcError {
+    #[error("connection error: {0}")]
+    Connection(String),
+
+    #[error("encoding error: {0}")]
+    Encode(String),
+
+    #[error("decoding error: {0}")]
+    Decode(String),
+
+    #[error("server returned error status {status:?}: {message}")]
+    Server {
+        status: RpcStatus,
+        message: String,
+    },
+
+    #[error("request timed out")]
+    Timeout,
+
+    #[error("stream closed unexpectedly")]
+    StreamClosed,
+
+    #[error("payload too large: {size} bytes (max {max})")]
+    PayloadTooLarge { size: usize, max: usize },
+}