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.

CLAUDE.md

@@ -121,6 +121,7 @@ Read [`docs/`](docs/) before changing behavior. Order:
 6. [docs/tech-stack.md](docs/tech-stack.md) — libraries, permissive-license rule, tooling
 7. [docs/deployment.md](docs/deployment.md) — zero-config self-host (Docker / binary / source)
 8. [docs/roadmap.md](docs/roadmap.md) — milestones + resolved decisions
+9. [docs/building.md](docs/building.md) — what each CMake preset is for + manual server/`vccli` testing
 
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