chore: rename quicproquo → quicprochat in Rust workspace

Rename all crate directories, package names, binary names, proto
package/module paths, ALPN strings, env var prefixes, config filenames,
mDNS service names, and plugin ABI symbols from quicproquo/qpq to
quicprochat/qpc.

proto/qpc/v1/push.proto

@@ -0,0 +1,49 @@
+syntax = "proto3";
+package qpc.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;
+}