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>

clients/windows/README.md

@@ -0,0 +1,18 @@
+# Windows client — placeholder
+
+Built in **M4** (see [`docs/roadmap.md`](../../docs/roadmap.md)). C# / .NET 8+, consuming
+`libvoicecat` through the C ABI ([`core/include/voicecat.h`](../../core/include/voicecat.h)).
+
+Planned shape (see [`docs/architecture.md`](../../docs/architecture.md) §4 and
+[`docs/tech-stack.md`](../../docs/tech-stack.md) §2):
+
+- A .NET solution with a P/Invoke interop layer over the C ABI using **`LibraryImport`**
+  (source-generated, .NET 7+). Build `libvoicecat` as a **shared library**
+  (`-DVOICECAT_BUILD_SHARED=ON`) so the DLL sits beside the app.
+- The `on_event` callback marshaled as a function pointer (`[UnmanagedCallersOnly]`) to avoid
+  delegate-lifetime issues; keep the interface "chunky" to minimize managed↔native crossings.
+- UI in **WinUI 3** (most native) or **Avalonia** (if a single C# desktop UI is wanted later).
+- Audio (capture/playback, WASAPI loopback for `SCREEN_AUDIO`) is handled inside the core; the
+  C# layer only drives device selection, meters, and the VAD/PTT + per-user NR controls.
+
+Nothing here yet — the core must reach M2 (working voice) before the GUI is worth building.