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.

crates/quicproquo-sdk/src/events.rs

@@ -0,0 +1,56 @@
+//! Client event system — real-time notifications from the SDK.
+
+/// Events emitted by the SDK to the UI layer.
+#[derive(Debug, Clone)]
+pub enum ClientEvent {
+    /// Successfully connected to the server.
+    Connected,
+
+    /// Disconnected from the server.
+    Disconnected { reason: String },
+
+    /// Authentication succeeded.
+    Authenticated { username: String },
+
+    /// A new message was received in a conversation.
+    MessageReceived {
+        conversation_id: [u8; 16],
+        sender_key: Vec<u8>,
+        sender_name: Option<String>,
+        body: String,
+        timestamp_ms: u64,
+    },
+
+    /// A message was sent successfully.
+    MessageSent {
+        conversation_id: [u8; 16],
+        seq: u64,
+    },
+
+    /// A new conversation was created or discovered.
+    ConversationCreated {
+        conversation_id: [u8; 16],
+        display_name: String,
+    },
+
+    /// A member was added to a group conversation.
+    MemberAdded {
+        conversation_id: [u8; 16],
+        member_key: Vec<u8>,
+    },
+
+    /// A member was removed from a group conversation.
+    MemberRemoved {
+        conversation_id: [u8; 16],
+        member_key: Vec<u8>,
+    },
+
+    /// Server-push event received.
+    PushEvent {
+        event_type: u16,
+        payload: Vec<u8>,
+    },
+
+    /// An error occurred in the background.
+    Error { message: String },
+}