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.

crates/quicprochat-core/proto/chat_message.proto

@@ -0,0 +1,21 @@
+syntax = "proto3";
+package quicprochat.bench;
+
+// Equivalent to the Envelope struct in delivery.capnp
+message Envelope {
+  uint64 seq = 1;
+  bytes data = 2;
+}
+
+// Equivalent to a chat message payload (app_message.rs Chat variant)
+message ChatMessage {
+  bytes message_id = 1;   // 16 bytes
+  string body = 2;        // UTF-8 text
+  uint64 timestamp_ms = 3;
+  bytes sender_key = 4;   // 32 bytes Ed25519 public key
+}
+
+// Batch fetch response (equivalent to fetch returning List(Envelope))
+message FetchResponse {
+  repeated Envelope payloads = 1;
+}