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.

server/src/main.cpp

@@ -1,8 +1,10 @@
 /*
- * voicecat-server entry point (M0 skeleton).
+ * voicecat-server entry point.
  *
- * Goal (docs/deployment.md): one command, zero config, encrypted + listening. Today it
- * parses a few flags and prints what it would do.
+ * Goal (docs/deployment.md): one command, zero config, encrypted + listening. Parses flags
+ * (see docs/building.md for the full list + manual-testing walkthrough) and hands off to
+ * Server::run() (server.cpp), which really does generate the identity/cert, open the SQLite
+ * store, and bind the TCP/TLS + UDP listeners.
  */
 #include <cstdio>
 #include <cstring>