fix: harden DGR-002 protocol bounds

packages/node/native/proto/shard_runtime.proto

@@ -94,9 +94,10 @@ message Checksum {
 
 // One slice of a tensor's wire payload.
 //
-// Fragments bound the size of a single gRPC message. `byte_offset` lets a
-// receiver verify the fragments tile the body exactly — no hole, no overlap —
-// instead of trusting arrival order.
+// Fragments bound individual payload pieces and make coverage independently
+// verifiable. The negotiated `FlowControl.max_chunk_bytes` bounds the enclosing
+// serialized stream message; `byte_offset` proves the pieces tile the tensor
+// exactly — no hole, no overlap — instead of trusting arrival order.
 //
 // Offsets and payloads describe the *wire* body, which is the compressed frame
 // when the parent tensor declares a compression. A zstd frame is not decodable