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/blob.proto

@@ -0,0 +1,29 @@
+syntax = "proto3";
+package qpc.v1;
+
+// Blob upload/download (2 methods).
+// Method IDs: 600-601.
+
+message UploadBlobRequest {
+  bytes blob_hash = 1;
+  bytes chunk = 2;
+  uint64 offset = 3;
+  uint64 total_size = 4;
+  string mime_type = 5;
+}
+
+message UploadBlobResponse {
+  bytes blob_id = 1;
+}
+
+message DownloadBlobRequest {
+  bytes blob_id = 1;
+  uint64 offset = 2;
+  uint32 length = 3;
+}
+
+message DownloadBlobResponse {
+  bytes chunk = 1;
+  uint64 total_size = 2;
+  string mime_type = 3;
+}