WIP: add OPAQUE password-authenticated key exchange

Add opaque-ke (v4, ristretto255) for password-based registration and
login. Extend NodeService schema with opaqueRegisterStart/Finish and
opaqueLoginStart/Finish RPCs. Add Store trait methods for OPAQUE server
setup and user records. Initial e2e integration test scaffolding.

Note: FileBackedStore does not yet implement the new Store trait
methods — server compilation is temporarily broken.

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

crates/quicnprotochat-core/Cargo.toml

@@ -18,6 +18,9 @@ rand                = { workspace = true }
 # Crypto — post-quantum hybrid KEM (M7)
 ml-kem              = { workspace = true }
 
+# Crypto — OPAQUE password-authenticated key exchange
+opaque-ke           = { workspace = true }
+
 # Crypto — MLS (M2)
 openmls             = { workspace = true }
 openmls_rust_crypto = { workspace = true }