scaffold: M0 skeleton + agent onboarding (build, architecture, progress)

Turn the design into a buildable, dependency-free M0 skeleton plus the
onboarding layer so a new agent can pick up instantly.

Build system:
- CMake + CMakePresets (dev = no deps; server-release = vcpkg) + vcpkg.json
- Skeleton builds with just a C++20 compiler; deps stay off until needed
- .gitattributes (LF), .gitignore, .clang-format

Core (libvoicecat):
- core/include/voicecat.h: full C ABI (the client/server contract), stubbed
- core/proto/voicecat.proto: control-plane wire format, matches docs/protocol.md
- src/{net,crypto,codec,protocol,session,audio,core}: subsystem stubs that
  return VC_ERR_NOT_IMPLEMENTED, each pointing to its design doc
- server/ (voicecat-server) and tools/vccli/ link the core
- tests/: CTest smoke test asserting the C ABI contract (behavior, not just build)
- clients/{apple,windows}: M4 placeholders

Onboarding for agents:
- CLAUDE.md: hub — build/test commands, architecture at a glance, doc map, rules
- AGENTS.md: working method (behavior-driven; clean compile is the floor not the goal)
- PROGRESS.md: living tracker — M0 done, M1 task checklist, "where we left off"

Verified: cmake --preset dev && cmake --build --preset dev && ctest --preset dev → green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/src/audio/audio_engine.cpp

@@ -0,0 +1,8 @@
+#include "audio/audio_engine.h"
+
+namespace voicecat::audio {
+
+// M0 stub. Capture/playback (miniaudio), APM DSP, jitter buffer, and mixer land in M2/M3.
+// See docs/voice.md §8–11.
+
+}  // namespace voicecat::audio

core/src/audio/audio_engine.h

@@ -0,0 +1,40 @@
+/*
+ * audio/audio_engine.h — capture/playback + DSP + jitter buffer + mixer.
+ *
+ * Design: docs/voice.md §8–11. Real-time path:
+ *   capture(miniaudio) → APM(AEC/NS/AGC/VAD, send-side) → Opus encode → ...
+ *   ... → Opus decode → per-user recv NS (listener-chosen) → gain/mute → mix → playback
+ *
+ * REAL-TIME RULE: audio-callback threads never allocate, lock, or block (architecture.md §3).
+ *
+ * STATUS: M0 stub.
+ */
+#ifndef VOICECAT_AUDIO_AUDIO_ENGINE_H
+#define VOICECAT_AUDIO_AUDIO_ENGINE_H
+
+#include <cstdint>
+
+namespace voicecat::audio {
+
+// Adaptive per-ssrc jitter buffer (voice.md §5). TODO(M2).
+class JitterBuffer {
+   public:
+    uint32_t target_depth_ms() const { return target_depth_ms_; }
+
+   private:
+    uint32_t target_depth_ms_ = 40;
+};
+
+// Owns miniaudio capture/playback, the APM instances, codecs, jitter buffers, and the mixer.
+class AudioEngine {
+   public:
+    // TODO(M2): start/stop capture+playback; push/pull frames via lock-free ring buffers.
+    bool running() const { return running_; }
+
+   private:
+    bool running_ = false;
+};
+
+}  // namespace voicecat::audio
+
+#endif  // VOICECAT_AUDIO_AUDIO_ENGINE_H