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/net/transport.cpp

@@ -0,0 +1,8 @@
+#include "net/transport.h"
+
+namespace voicecat::net {
+
+// M0 stub. Subsystem brought up in M1 (TCP/TLS) and M2 (UDP). See docs/protocol.md,
+// docs/voice.md, and AGENTS.md "Suggested first steps".
+
+}  // namespace voicecat::net

core/src/net/transport.h

@@ -0,0 +1,39 @@
+/*
+ * net/transport.h — TCP control channel + UDP media channel.
+ *
+ * Design: docs/architecture.md (Net thread), docs/protocol.md §1 (framing), docs/voice.md §2
+ * (UDP frame). Implementation will use standalone Asio (one reactor) for sockets/timers.
+ *
+ * STATUS: M0 stub — interfaces only, no Asio yet.
+ */
+#ifndef VOICECAT_NET_TRANSPORT_H
+#define VOICECAT_NET_TRANSPORT_H
+
+#include <cstdint>
+#include <string>
+
+namespace voicecat::net {
+
+// Length-prefixed [u32 length][payload] framing over a TLS 1.3 byte stream (protocol.md §1).
+class TcpControlChannel {
+   public:
+    // TODO(M1): connect(host, port), TLS handshake, send/recv framed Envelopes.
+    bool connected() const { return connected_; }
+
+   private:
+    bool connected_ = false;
+};
+
+// UDP media channel: encrypted voice frames (voice.md §2), bound to a session via token.
+class UdpMediaChannel {
+   public:
+    // TODO(M2): bind, send/recv AEAD-sealed voice frames, keepalive.
+    bool bound() const { return bound_; }
+
+   private:
+    bool bound_ = false;
+};
+
+}  // namespace voicecat::net
+
+#endif  // VOICECAT_NET_TRANSPORT_H