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>

tests/CMakeLists.txt

@@ -0,0 +1,8 @@
+# Tests use plain asserts + exit codes for now (no framework dependency in the skeleton).
+# A real framework (e.g. Catch2/GoogleTest via vcpkg) can be added when VOICECAT_USE_VCPKG_DEPS
+# is on. Behavior tests — not just "it compiles" — are how milestones are judged (AGENTS.md).
+
+add_executable(test_smoke test_smoke.cpp)
+target_link_libraries(test_smoke PRIVATE voicecat::voicecat)
+target_compile_features(test_smoke PRIVATE cxx_std_20)
+add_test(NAME smoke COMMAND test_smoke)

tests/test_smoke.cpp

@@ -0,0 +1,65 @@
+/*
+ * test_smoke — verifies the core links and the C ABI behaves as specified for M0.
+ *
+ * This is intentionally a behavior test, not a "does it compile" check: it asserts the
+ * documented contract (version present, handle lifecycle, invalid-arg guards, and that
+ * unimplemented calls report VC_ERR_NOT_IMPLEMENTED rather than crashing).
+ */
+#include <cstdio>
+#include <cstring>
+
+#include "voicecat.h"
+
+static int g_failures = 0;
+
+#define CHECK(cond)                                                       \
+    do {                                                                  \
+        if (!(cond)) {                                                    \
+            std::printf("FAIL: %s (%s:%d)\n", #cond, __FILE__, __LINE__); \
+            ++g_failures;                                                 \
+        }                                                                 \
+    } while (0)
+
+int main() {
+    // Version + result strings are always available.
+    CHECK(vc_version_string() != nullptr);
+    CHECK(std::strlen(vc_version_string()) > 0);
+    CHECK(std::strcmp(vc_result_string(VC_OK), "ok") == 0);
+    CHECK(std::strcmp(vc_result_string(VC_ERR_NOT_IMPLEMENTED), "not implemented") == 0);
+
+    // Null-config create is rejected; valid create yields a handle.
+    vc_callbacks cb{};
+    CHECK(vc_client_create(nullptr, cb) == nullptr);
+
+    vc_config cfg{};
+    cfg.client_name = "test";
+    cfg.client_version = "0";
+    cfg.log_level = VC_LOG_OFF;
+    vc_client* c = vc_client_create(&cfg, cb);
+    CHECK(c != nullptr);
+
+    // Invalid-arg guards on the ABI.
+    CHECK(vc_connect(nullptr, "h", 1) == VC_ERR_INVALID_ARG);
+    CHECK(vc_connect(c, nullptr, 1) == VC_ERR_INVALID_ARG);
+    CHECK(vc_send_text(c, VC_TEXT_CHANNEL, 0, nullptr) == VC_ERR_INVALID_ARG);
+
+    // Unimplemented subsystems report NOT_IMPLEMENTED (not a crash) in the M0 skeleton.
+    CHECK(vc_connect(c, "127.0.0.1", 8384) == VC_ERR_NOT_IMPLEMENTED);
+    CHECK(vc_authenticate_guest(c, "nick") == VC_ERR_NOT_IMPLEMENTED);
+    CHECK(vc_join_channel(c, 1, nullptr) == VC_ERR_NOT_IMPLEMENTED);
+
+    vc_device_list dl;
+    CHECK(vc_list_devices(c, VC_DEVICE_INPUT, &dl) == VC_ERR_NOT_IMPLEMENTED);
+    CHECK(dl.count == 0);
+    vc_free_device_list(&dl);
+
+    vc_client_destroy(c);
+    vc_client_destroy(nullptr);  // must be safe
+
+    if (g_failures == 0) {
+        std::printf("smoke: all checks passed\n");
+        return 0;
+    }
+    std::printf("smoke: %d failure(s)\n", g_failures);
+    return 1;
+}