docs(sdk): add comprehensive SDK documentation and wire format reference

Covers all official SDKs (Rust, Go, Python, TypeScript, C FFI),
the v2 wire format with method ID tables, authentication flow,
and a build-your-own-SDK guide with implementation checklist.

docs/sdk/rust.md

@@ -0,0 +1,63 @@
+# Rust SDK
+
+The Rust client is the reference implementation, located in `crates/quicproquo-client/`.
+
+## Installation
+
+Add to your `Cargo.toml`:
+
+```toml
+[dependencies]
+quicproquo-client = { path = "crates/quicproquo-client" }
+```
+
+## Connection
+
+The Rust client connects directly over QUIC with Cap'n Proto RPC:
+
+```rust
+use quicproquo_client::{cmd_health, cmd_login, cmd_send, connect_node};
+
+// Health check
+cmd_health("127.0.0.1:5001", &ca_cert_path, "localhost").await?;
+
+// Login via OPAQUE
+cmd_login(
+    "127.0.0.1:5001", &ca_cert_path, "localhost",
+    "alice", "password123",
+    None,                    // identity_key_hex
+    Some(&state_path),       // state persistence
+    None,                    // state_password
+).await?;
+```
+
+## Key Features
+
+- Full MLS (RFC 9420) group encryption
+- Hybrid post-quantum KEM (X25519 + ML-KEM-768)
+- OPAQUE authentication with zeroizing credential storage
+- SQLCipher local state with Argon2id key derivation
+- Sealed sender metadata protection
+- v2 QUIC + Protobuf transport (via `quicproquo-sdk` crate)
+
+## v2 SDK Crate
+
+The `quicproquo-sdk` crate provides the higher-level v2 API:
+
+```rust
+use quicproquo_sdk::QpqClient;
+
+let client = QpqClient::connect("127.0.0.1:5001", &tls_config).await?;
+let health = client.health().await?;
+```
+
+## Crate Structure
+
+| Crate | Purpose |
+|---|---|
+| `quicproquo-core` | Crypto primitives, MLS, hybrid KEM |
+| `quicproquo-proto` | Protobuf + Cap'n Proto generated types |
+| `quicproquo-rpc` | QUIC RPC framework (framing, dispatch) |
+| `quicproquo-sdk` | High-level client SDK |
+| `quicproquo-client` | CLI/TUI client application |
+| `quicproquo-ffi` | C FFI bindings |