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.

proto/qpq/v1/auth.proto

@@ -0,0 +1,43 @@
+syntax = "proto3";
+package qpq.v1;
+
+// OPAQUE registration + login (4 methods).
+// Method IDs: 100-103.
+
+message OpaqueRegisterStartRequest {
+  string username = 1;
+  bytes request = 2;
+}
+
+message OpaqueRegisterStartResponse {
+  bytes response = 1;
+}
+
+message OpaqueRegisterFinishRequest {
+  string username = 1;
+  bytes upload = 2;
+  bytes identity_key = 3;
+}
+
+message OpaqueRegisterFinishResponse {
+  bool success = 1;
+}
+
+message OpaqueLoginStartRequest {
+  string username = 1;
+  bytes request = 2;
+}
+
+message OpaqueLoginStartResponse {
+  bytes response = 1;
+}
+
+message OpaqueLoginFinishRequest {
+  string username = 1;
+  bytes finalization = 2;
+  bytes identity_key = 3;
+}
+
+message OpaqueLoginFinishResponse {
+  bytes session_token = 1;
+}