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/crypto/crypto.h

@@ -0,0 +1,40 @@
+/*
+ * crypto/crypto.h — TLS 1.3 (mbedTLS) and the media AEAD (libsodium).
+ *
+ * Design: docs/security.md. Control channel = TLS 1.3. Media = keys exported from the TLS
+ * session (RFC 5705 / 8446) + per-frame ChaCha20-Poly1305 with a counter nonce and a
+ * sliding-window replay filter. Encryption is MANDATORY — never add a plaintext path.
+ *
+ * STATUS: M0 stub.
+ */
+#ifndef VOICECAT_CRYPTO_CRYPTO_H
+#define VOICECAT_CRYPTO_CRYPTO_H
+
+#include <cstddef>
+#include <cstdint>
+
+namespace voicecat::crypto {
+
+// TLS 1.3 endpoint wrapper (mbedTLS). Provides the keying-material exporter that seeds
+// MediaCrypto, so the UDP path inherits the authenticated control session's trust.
+class TlsContext {
+   public:
+    // TODO(M1): client/server handshake; read/write; export_keying_material(label,...).
+};
+
+// Per-frame media encryption. Abstracted so the backend (exported-key AEAD now; a DTLS 1.3
+// backend later, if a permissive impl matures) is swappable without touching voice code.
+class MediaCrypto {
+   public:
+    virtual ~MediaCrypto() = default;
+    // seal/open one voice frame; `aad` carries the routable header fields (e.g. ssrc).
+    // Returns bytes written, or -1 on failure (replay/auth). TODO(M2).
+    virtual long seal(const uint8_t* plain, size_t len, const uint8_t* aad, size_t aad_len,
+                      uint8_t* out, size_t out_cap) = 0;
+    virtual long open(const uint8_t* sealed, size_t len, const uint8_t* aad, size_t aad_len,
+                      uint8_t* out, size_t out_cap) = 0;
+};
+
+}  // namespace voicecat::crypto
+
+#endif  // VOICECAT_CRYPTO_CRYPTO_H