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/delivery.proto

@@ -0,0 +1,72 @@
+syntax = "proto3";
+package qpq.v1;
+
+// Delivery service: enqueue, fetch, peek, ack, batch (6 methods).
+// Method IDs: 200-205.
+
+message Envelope {
+  uint64 seq = 1;
+  bytes data = 2;
+}
+
+message EnqueueRequest {
+  bytes recipient_key = 1;
+  bytes payload = 2;
+  bytes channel_id = 3;
+  uint32 ttl_secs = 4;
+}
+
+message EnqueueResponse {
+  uint64 seq = 1;
+  bytes delivery_proof = 2;
+}
+
+message FetchRequest {
+  bytes recipient_key = 1;
+  bytes channel_id = 2;
+  uint32 limit = 3;
+}
+
+message FetchResponse {
+  repeated Envelope payloads = 1;
+}
+
+message FetchWaitRequest {
+  bytes recipient_key = 1;
+  bytes channel_id = 2;
+  uint64 timeout_ms = 3;
+  uint32 limit = 4;
+}
+
+message FetchWaitResponse {
+  repeated Envelope payloads = 1;
+}
+
+message PeekRequest {
+  bytes recipient_key = 1;
+  bytes channel_id = 2;
+  uint32 limit = 3;
+}
+
+message PeekResponse {
+  repeated Envelope payloads = 1;
+}
+
+message AckRequest {
+  bytes recipient_key = 1;
+  bytes channel_id = 2;
+  uint64 seq_up_to = 3;
+}
+
+message AckResponse {}
+
+message BatchEnqueueRequest {
+  repeated bytes recipient_keys = 1;
+  bytes payload = 2;
+  bytes channel_id = 3;
+  uint32 ttl_secs = 4;
+}
+
+message BatchEnqueueResponse {
+  repeated uint64 seqs = 1;
+}