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/user.proto

@@ -0,0 +1,22 @@
+syntax = "proto3";
+package qpc.v1;
+
+// User resolve + identity (2 methods).
+// Method IDs: 500-501.
+
+message ResolveUserRequest {
+  string username = 1;
+}
+
+message ResolveUserResponse {
+  bytes identity_key = 1;
+  bytes inclusion_proof = 2;
+}
+
+message ResolveIdentityRequest {
+  bytes identity_key = 1;
+}
+
+message ResolveIdentityResponse {
+  string username = 1;
+}