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/federation.proto

@@ -0,0 +1,65 @@
+syntax = "proto3";
+package qpq.v1;
+
+// Federation relay + proxy (6 methods).
+// Method IDs: 900-905.
+
+message FederationAuth {
+  string origin = 1;
+}
+
+message RelayEnqueueRequest {
+  bytes recipient_key = 1;
+  bytes payload = 2;
+  bytes channel_id = 3;
+  FederationAuth auth = 4;
+}
+
+message RelayEnqueueResponse {
+  uint64 seq = 1;
+}
+
+message RelayBatchEnqueueRequest {
+  repeated bytes recipient_keys = 1;
+  bytes payload = 2;
+  bytes channel_id = 3;
+  FederationAuth auth = 4;
+}
+
+message RelayBatchEnqueueResponse {
+  repeated uint64 seqs = 1;
+}
+
+message ProxyFetchKeyPackageRequest {
+  bytes identity_key = 1;
+  FederationAuth auth = 2;
+}
+
+message ProxyFetchKeyPackageResponse {
+  bytes package = 1;
+}
+
+message ProxyFetchHybridKeyRequest {
+  bytes identity_key = 1;
+  FederationAuth auth = 2;
+}
+
+message ProxyFetchHybridKeyResponse {
+  bytes hybrid_public_key = 1;
+}
+
+message ProxyResolveUserRequest {
+  string username = 1;
+  FederationAuth auth = 2;
+}
+
+message ProxyResolveUserResponse {
+  bytes identity_key = 1;
+}
+
+message FederationHealthRequest {}
+
+message FederationHealthResponse {
+  string status = 1;
+  string server_domain = 2;
+}