docs: add protocol flow overview

Author: mikesmitty

src/machine/usb/msc/msc.go

@@ -175,6 +175,33 @@ func rxHandler(b []byte) {
 	}
 }
 
+/*
+	Connection Happy Path Overview:
+
+0. MSC starts out in mscStateCmd status.
+
+1. Host sends CBW (Command Block Wrapper) packet to MSC.
+  - CBW contains the SCSI command to be executed, the length of the data to be transferred, etc.
+
+2. MSC receives CBW.
+  - CBW is validated and saved.
+  - State is changed to mscStateData.
+  - MSC routes the command to the appropriate SCSI command handler.
+
+3. The MSC SCSI command handler responds with the initial data packet (if applicable).
+  - If no data packet is needed, state is changed to mscStateStatus and step 4 is skipped.
+
+4. The host acks the data packet and MSC calls m.scsiDataTransfer() to continue sending (or
+receiving) data.
+  - This cycle continues until all data requested in the CBW is sent/received.
+  - State is changed to mscStateStatus.
+  - MSC waits for the host to ACK the final data packet.
+
+5. MSC then sends a CSW (Command Status Wrapper) to the host to report the final status of the
+command execution and moves to mscStateStatusSent.
+
+6. The host ACKs the CSW and the MSC moves back to mscStateCmd, waiting for the next CBW.
+*/
 func (m *msc) run(b []byte, isEpOut bool) {
 	// TODO: Cleanup?
 	if m.prevState != m.state {