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/common.proto

@@ -0,0 +1,19 @@
+syntax = "proto3";
+package qpq.v1;
+
+// Common types shared across services.
+
+// Auth context included in authenticated RPC requests.
+// In v2, this is carried as QUIC connection-level state (session token),
+// not per-message. Included here for federation and internal use.
+message Auth {
+  bytes access_token = 1;
+  bytes device_id = 2;
+}
+
+// Account deletion.
+message DeleteAccountRequest {}
+
+message DeleteAccountResponse {
+  bool success = 1;
+}