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/opaque_auth.rs

@@ -0,0 +1,20 @@
+//! Shared OPAQUE (RFC 9497) cipher suite configuration.
+//!
+//! Both client and server import this module to ensure they use exactly
+//! the same cryptographic parameters during registration and login.
+
+use opaque_ke::CipherSuite;
+
+/// OPAQUE cipher suite for quicprochat.
+///
+/// - **OPRF**: Ristretto255 (curve25519-based, ~128-bit security)
+/// - **Key exchange**: Triple-DH (3DH) over Ristretto255 with SHA-512
+/// - **KSF**: Argon2id (memory-hard key stretching)
+pub struct OpaqueSuite;
+
+impl CipherSuite for OpaqueSuite {
+    type OprfCs = opaque_ke::Ristretto255;
+    type KeyExchange =
+        opaque_ke::key_exchange::tripledh::TripleDh<opaque_ke::Ristretto255, sha2::Sha512>;
+    type Ksf = argon2::Argon2<'static>;
+}