engine: begin the liboakengine C ABI facade with the IPC subsystem

- oakengine/export.h establishes the OAKENGINE_API visibility macros;
  include/oakengine/ipc.h is the first pure-C surface (41 functions:
  shm, frame slot pool, and the worker IPC messages as POD<->JSON
  build/parse), implemented in engine/src/capi/
- the IPC implementations move to engine/src/oliveimpl (namespace
  olive::engine::internal::ipc); engine/render/ipc/*.h are rebuilt as
  same-name/same-API wrapper classes forwarding across the C boundary
- FrameSlotMeta is shared with the C header verbatim so the app/worker
  wire format (v1) is bit-identical; static_asserts pin sizeof and
  field offsets
- spscringbuffer.h moves to include/oakengine/ as an inline-only
  header (no symbols, not ABI)
- new pure-C test oakengine_ipc_test (make_oakengine_test, no GL)
  covers shm, frame pool, message round-trips and the layout asserts;
  full gtest suite stays green (1986 tests)

engine/include/oakengine/export.h

@@ -0,0 +1,45 @@
+/***
+
+  Oak - Non-Linear Video Editor
+  Copyright (C) 2026 Oak Team
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#ifndef OAKENGINE_EXPORT_H
+#define OAKENGINE_EXPORT_H
+
+/**
+ * @file export.h
+ * @brief Symbol visibility macros for liboakengine
+ *
+ * liboakengine is transitioning to a pure C ABI: every public C function is
+ * declared with OAKENGINE_API. While the migration is in flight the library
+ * is still built with default symbol visibility, so the legacy C++ symbols
+ * remain exported alongside the C ABI.
+ */
+#if defined(_WIN32) || defined(__CYGWIN__)
+  #ifdef OAKENGINE_BUILD
+    #define OAKENGINE_API __declspec(dllexport)
+  #else
+    #define OAKENGINE_API __declspec(dllimport)
+  #endif
+#elif defined(__GNUC__) || defined(__clang__)
+  #define OAKENGINE_API __attribute__((visibility("default")))
+#else
+  #define OAKENGINE_API
+#endif
+
+#endif /* OAKENGINE_EXPORT_H */