feat: v2 Phase 1 — foundation, proto schemas, RPC framework, SDK skeleton

New workspace structure with 9 crates. Adds:

- proto/qpq/v1/*.proto: 11 protobuf schemas covering all 33 RPC methods
- quicproquo-proto: dual codegen (capnp legacy + prost v2)
- quicproquo-rpc: QUIC RPC framework (framing, server, client, middleware)
- quicproquo-sdk: client SDK (QpqClient, events, conversation store)
- quicproquo-server/domain/: protocol-agnostic domain types and services
- justfile: build commands

Wire format: [method_id:u16][req_id:u32][len:u32][protobuf] per QUIC stream.
All 151 existing tests pass. Backward compatible with v1 capnp code.

crates/quicproquo-server/src/domain/mod.rs

@@ -0,0 +1,10 @@
+//! Domain types and service logic — protocol-agnostic.
+//!
+//! These types define the server's business logic independently of any
+//! serialization format (Cap'n Proto, Protobuf). RPC handlers translate
+//! wire-format messages into these types, call service functions, and
+//! translate the results back.
+
+pub mod types;
+pub mod auth;
+pub mod delivery;