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-core/src/error.rs

@@ -0,0 +1,38 @@
+//! Error types for `quicprochat-core`.
+
+use thiserror::Error;
+
+/// Errors produced by core cryptographic and MLS operations.
+#[derive(Debug, Error)]
+pub enum CoreError {
+    /// Cap'n Proto serialisation or deserialisation failed.
+    #[cfg(feature = "native")]
+    #[error("Cap'n Proto error: {0}")]
+    Capnp(#[from] capnp::Error),
+
+    /// An MLS operation failed (string description).
+    ///
+    /// Preserved for backward compatibility. Prefer [`CoreError::MlsError`]
+    /// for new code that wraps typed openmls errors.
+    #[error("MLS error: {0}")]
+    Mls(String),
+
+    /// An MLS operation failed (typed, boxed error).
+    ///
+    /// Wraps the underlying openmls error so callers can downcast to specific
+    /// error types when needed.
+    #[error("MLS error: {0}")]
+    MlsError(Box<dyn std::error::Error + Send + Sync>),
+
+    /// A hybrid KEM (X25519 + ML-KEM-768) operation failed.
+    #[error("hybrid KEM error: {0}")]
+    HybridKem(#[from] crate::hybrid_kem::HybridKemError),
+
+    /// IO or persistence failure.
+    #[error("io error: {0}")]
+    Io(String),
+
+    /// Application message (rich payload) parse or serialisation error.
+    #[error("app message: {0}")]
+    AppMessage(String),
+}