fix(cursor): address third-round review feedback for protobuf module

HIGH PRIORITY FIXES:
- Extract shared buffer parsing logic into parseProtobufFrames generator
  method (DRY violation fix) - both JSON and SSE transformers now use
  common frame parsing loop, eliminating ~60% code duplication
- Use COMPRESS_FLAG constants instead of hardcoded 0x01/0x02/0x03 in
  parseConnectRPCFrame for better maintainability

MEDIUM PRIORITY FIXES:
- Return empty buffer on gzip decompression failure (prevents silent
  data corruption) - ALREADY FIXED
- Add debug warning for unknown message roles in convertMessages
- Create GitHub issue #535 for FIELD namespace refactoring follow-up
- Add test coverage: transformProtobufToSSE, error paths, unknown roles

LOW PRIORITY FIXES:
- Rename checksum test to clarify timestamp granularity (~16 min)
- Fix debug log function name in SSE transformer - FIXED BY REFACTOR
- Add comment to TOOL_RESULT_RESULT field documenting future use

All tests pass (1593 pass, 0 fail)
All validation checks pass (typecheck + lint + format + tests)

src/cursor/cursor-protobuf-schema.ts

@@ -75,7 +75,7 @@ export const FIELD = {
   TOOL_RESULT_NAME: 2,
   TOOL_RESULT_INDEX: 3,
   TOOL_RESULT_RAW_ARGS: 5,
-  TOOL_RESULT_RESULT: 8,
+  TOOL_RESULT_RESULT: 8, // Reserved for future tool result parsing
 
   // ===== Model =====
   MODEL_NAME: 1,