feat: add post-quantum hybrid KEM + SQLCipher persistence

Feature 1 — Post-Quantum Hybrid KEM (X25519 + ML-KEM-768):
- Create hybrid_kem.rs with keygen, encrypt, decrypt + 11 unit tests
- Wire format: version(1) | x25519_eph_pk(32) | mlkem_ct(1088) | nonce(12) | ct
- Add uploadHybridKey/fetchHybridKey RPCs to node.capnp schema
- Server: hybrid key storage in FileBackedStore + RPC handlers
- Client: hybrid keypair in StoredState, auto-wrap/unwrap in send/recv/invite/join
- demo-group runs full hybrid PQ envelope round-trip

Feature 2 — SQLCipher Persistence:
- Extract Store trait from FileBackedStore API
- Create SqlStore (rusqlite + bundled-sqlcipher) with encrypted-at-rest SQLite
- Schema: key_packages, deliveries, hybrid_keys tables with indexes
- Server CLI: --store-backend=sql, --db-path, --db-key flags
- 5 unit tests for SqlStore (FIFO, round-trip, upsert, channel isolation)

Also includes: client lib.rs refactor, auth config, TOML config file support,
mdBook documentation, and various cleanups by user.

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

crates/quicnprotochat-core/src/identity.rs

@@ -1,11 +1,8 @@
 //! Ed25519 identity keypair for MLS credentials and AS registration.
 //!
-//! # Relationship to the Noise keypair
-//!
-//! The X25519 [`NoiseKeypair`](crate::NoiseKeypair) is the transport-layer
-//! static key used in the Noise_XX handshake. The Ed25519 [`IdentityKeypair`]
-//! is the long-term identity key embedded in MLS `BasicCredential`s. The two
-//! keys serve different roles and must not be confused.
+//! The [`IdentityKeypair`] is the long-term identity key embedded in MLS
+//! `BasicCredential`s. It is used for signing MLS messages and as the
+//! indexing key for the Authentication Service.
 //!
 //! # Zeroize
 //!