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/push.proto

@@ -0,0 +1,49 @@
+syntax = "proto3";
+package qpq.v1;
+
+// Server-push event types (sent on QUIC uni-streams).
+// Event type IDs: 1000+.
+
+// Wrapper for a push event.
+message PushEvent {
+  oneof event {
+    NewMessage new_message = 1;
+    TypingIndicator typing = 2;
+    PresenceUpdate presence = 3;
+    GroupMembershipChange membership = 4;
+  }
+}
+
+message NewMessage {
+  bytes channel_id = 1;
+  bytes sender_key = 2;
+  uint64 seq = 3;
+  bytes payload = 4;
+  uint64 timestamp_ms = 5;
+}
+
+message TypingIndicator {
+  bytes channel_id = 1;
+  bytes sender_key = 2;
+  bool is_typing = 3;
+}
+
+message PresenceUpdate {
+  bytes identity_key = 1;
+  bool online = 2;
+  uint64 last_seen_ms = 3;
+}
+
+message GroupMembershipChange {
+  bytes channel_id = 1;
+  bytes actor_key = 2;
+  bytes target_key = 3;
+  MembershipAction action = 4;
+}
+
+enum MembershipAction {
+  MEMBERSHIP_ACTION_UNSPECIFIED = 0;
+  MEMBERSHIP_ACTION_ADDED = 1;
+  MEMBERSHIP_ACTION_REMOVED = 2;
+  MEMBERSHIP_ACTION_LEFT = 3;
+}