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/channel.proto

@@ -0,0 +1,14 @@
+syntax = "proto3";
+package qpc.v1;
+
+// Channel create (1 method).
+// Method ID: 400.
+
+message CreateChannelRequest {
+  bytes peer_key = 1;
+}
+
+message CreateChannelResponse {
+  bytes channel_id = 1;
+  bool was_new = 2;
+}