docs: add protobuf debugging tools note to Phase 2 design

Author: ga11221

docs/phase2/design-notes.md

@@ -64,6 +64,21 @@ Steady-state: 3 fds per partition (active segment only). Closed segments release
 
 ---
 
+## Debugging Tools (Phase 2)
+
+### Protobuf Visibility
+
+2PC adds Prepare/Commit/Abort message flows. Need visibility during development.
+
+Options:
+- Server flag `-log-protobuf` — dumps decoded requests/responses to stderr
+- Standalone TCP proxy (~100 lines Go) — sits between client and server, decodes frames, logs, forwards
+- `protoc --decode` — offline decoding of captured traffic
+
+**Decision**: build server flag + proxy during Phase 2. Not now.
+
+---
+
 ## Recovery Functions
 
 | Function | Purpose |