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/common.proto

@@ -0,0 +1,19 @@
+syntax = "proto3";
+package qpc.v1;
+
+// Common types shared across services.
+
+// Auth context included in authenticated RPC requests.
+// In v2, this is carried as QUIC connection-level state (session token),
+// not per-message. Included here for federation and internal use.
+message Auth {
+  bytes access_token = 1;
+  bytes device_id = 2;
+}
+
+// Account deletion.
+message DeleteAccountRequest {}
+
+message DeleteAccountResponse {
+  bool success = 1;
+}