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/voicecat.cpp

@@ -1,9 +1,11 @@
 /*
- * voicecat.cpp — C ABI implementation (M0 skeleton).
+ * voicecat.cpp — C ABI implementation.
  *
- * Lifecycle (create/destroy) and trivial accessors are real. Everything that needs a
- * subsystem (net/crypto/codec/protocol/session/audio) returns VC_ERR_NOT_IMPLEMENTED for
- * now and is the work of M1+ (see AGENTS.md / docs/roadmap.md).
+ * Lifecycle (create/destroy) and trivial accessors are always real. Everything else below
+ * just delegates to vc_client (core/src/core/client.cpp): under VOICECAT_HAS_NET
+ * (`m1-dev`/`server-release` — see docs/building.md) that's the real M1–M3 implementation;
+ * under the no-deps `dev` preset, client.cpp's `#else` branch returns VC_ERR_NOT_IMPLEMENTED
+ * for all of it, to keep that skeleton build green.
  */
 #include "voicecat.h"
 
@@ -48,7 +50,7 @@ vc_client* vc_client_create(const vc_config* cfg, vc_callbacks cb) {
 
 void vc_client_destroy(vc_client* c) { delete c; }
 
-/* ── Everything below delegates to the (stub) client. ─────────────────────── */
+/* ── Everything below delegates to vc_client (real or stub, per preset above). ───────── */
 
 vc_result vc_connect(vc_client* c, const char* host, uint16_t port) {
     if (c == nullptr || host == nullptr) return VC_ERR_INVALID_ARG;