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/blob.proto

@@ -0,0 +1,29 @@
+syntax = "proto3";
+package qpq.v1;
+
+// Blob upload/download (2 methods).
+// Method IDs: 600-601.
+
+message UploadBlobRequest {
+  bytes blob_hash = 1;
+  bytes chunk = 2;
+  uint64 offset = 3;
+  uint64 total_size = 4;
+  string mime_type = 5;
+}
+
+message UploadBlobResponse {
+  bytes blob_id = 1;
+}
+
+message DownloadBlobRequest {
+  bytes blob_id = 1;
+  uint64 offset = 2;
+  uint32 length = 3;
+}
+
+message DownloadBlobResponse {
+  bytes chunk = 1;
+  uint64 total_size = 2;
+  string mime_type = 3;
+}