docs: add build/manual-testing guide, fix stale M0 stub claims in headers

- docs/building.md: explains what each CMake preset (dev, m1-dev, m2-dev,
  server-release) is actually for, and how to build voicecat-server + vccli
  for manual testing. Linked from CLAUDE.md's doc index.
- core/include/voicecat.h, core/src/voicecat.cpp, core/src/protocol/protocol.h,
  server/src/main.cpp: doc-header comments still claimed M0-skeleton/stub
  behavior (VC_ERR_NOT_IMPLEMENTED everywhere, "prints what it would do",
  protobuf codegen "commented") that M1-M3 made real. Updated to describe
  current behavior, with the dev-preset stub fallback noted explicitly where
  it still applies.

core/src/protocol/protocol.h

@@ -6,7 +6,10 @@
  * request_id ↔ response, and dispatches to handlers. Media frames do NOT come through here
  * (they use the fixed binary header in voice.md §2).
  *
- * STATUS: M0 stub — protobuf codegen is wired in CMake (commented) and turned on in M1.
+ * STATUS: real. Protobuf codegen is on (core/CMakeLists.txt) for VOICECAT_HAS_NET builds
+ * (`m1-dev`/`server-release`); FrameCodec below is fully implemented and used by both the
+ * client (net/transport.h) and the server (conn_session.cpp). See protocol/envelope.h for the
+ * Envelope-level encode/decode that sits on top of this.
  */
 #ifndef VOICECAT_PROTOCOL_PROTOCOL_H
 #define VOICECAT_PROTOCOL_PROTOCOL_H