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-kt/src/error.rs

@@ -0,0 +1,16 @@
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum KtError {
+    #[error("leaf index {index} is out of range for tree size {tree_size}")]
+    IndexOutOfRange { index: u64, tree_size: u64 },
+
+    #[error("inclusion proof verification failed: root mismatch")]
+    RootMismatch,
+
+    #[error("serialisation error: {0}")]
+    Serialisation(String),
+
+    #[error("identity key is already revoked")]
+    AlreadyRevoked,
+}