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/federation.proto

@@ -0,0 +1,65 @@
+syntax = "proto3";
+package qpc.v1;
+
+// Federation relay + proxy (6 methods).
+// Method IDs: 900-905.
+
+message FederationAuth {
+  string origin = 1;
+}
+
+message RelayEnqueueRequest {
+  bytes recipient_key = 1;
+  bytes payload = 2;
+  bytes channel_id = 3;
+  FederationAuth auth = 4;
+}
+
+message RelayEnqueueResponse {
+  uint64 seq = 1;
+}
+
+message RelayBatchEnqueueRequest {
+  repeated bytes recipient_keys = 1;
+  bytes payload = 2;
+  bytes channel_id = 3;
+  FederationAuth auth = 4;
+}
+
+message RelayBatchEnqueueResponse {
+  repeated uint64 seqs = 1;
+}
+
+message ProxyFetchKeyPackageRequest {
+  bytes identity_key = 1;
+  FederationAuth auth = 2;
+}
+
+message ProxyFetchKeyPackageResponse {
+  bytes package = 1;
+}
+
+message ProxyFetchHybridKeyRequest {
+  bytes identity_key = 1;
+  FederationAuth auth = 2;
+}
+
+message ProxyFetchHybridKeyResponse {
+  bytes hybrid_public_key = 1;
+}
+
+message ProxyResolveUserRequest {
+  string username = 1;
+  FederationAuth auth = 2;
+}
+
+message ProxyResolveUserResponse {
+  bytes identity_key = 1;
+}
+
+message FederationHealthRequest {}
+
+message FederationHealthResponse {
+  string status = 1;
+  string server_domain = 2;
+}