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/render/renderworkerpool.h

@@ -32,9 +32,6 @@
 
 #include "codec/frame.h"
 #include "node/project/serializer/serializer.h"
-#include "render/ipc/frameslotpool.h"
-#include "render/ipc/ipcmessage.h"
-#include "render/ipc/sharedmemoryregion.h"
 #include "render/rendermanager.h"
 
 class QProcess;
@@ -42,6 +39,16 @@ class QProcess;
 namespace olive
 {
 
+// The worker IPC implementation lives behind the liboakengine C ABI facade
+// (src/oliveimpl/render/ipc). This header is consumed outside the engine
+// library, so it can only forward-declare the internal types used by private
+// method signatures; PooledWorker (which holds IPC objects by value) is
+// defined in the .cpp for the same reason.
+namespace engine::internal::ipc
+{
+class FrameSlotPool;
+}
+
 class Project;
 
 class RenderWorkerPool : public QThread {
@@ -90,24 +97,9 @@ private:
 		qint64 ticket_id = 0;
 	};
 
-	struct PooledWorker {
-		QProcess *process = nullptr;
-		QString loaded_graph_path;
-		qint64 last_used_ms = 0;
-		int use_count = 0;
-
-		// Persistent shared memory for this worker. Reusing regions across frames
-		// avoids the cost of creating/destroying large shm segments every render.
-		ipc::SharedMemoryRegion output_region;
-		ipc::FrameSlotPool output_pool;
-		size_t output_slot_bytes = 0;
-		QString output_shm_key;
-
-		ipc::SharedMemoryRegion input_region;
-		ipc::FrameSlotPool input_pool;
-		size_t input_slot_bytes = 0;
-		QString input_shm_key;
-	};
+	// Defined in the .cpp: holds the per-worker IPC shared-memory regions and
+	// frame slot pools (internal oliveimpl types) by value.
+	struct PooledWorker;
 
 	struct CachedGraph {
 		QString path;
@@ -124,7 +116,8 @@ private:
 					std::vector<std::unique_ptr<PooledWorker>> *local_pool);
 	JobResult process_job_attempt(const Job &job, int worker_index,
 								int attempt_index, PooledWorker *worker);
-	void finish_with_frame(RenderTicketPtr ticket, const ipc::FrameSlotPool &pool,
+	void finish_with_frame(RenderTicketPtr ticket,
+						 const engine::internal::ipc::FrameSlotPool &pool,
 						 uint32_t slot);
 	void cleanup_graph_file(const QString &path);
 	void add_graph_path_ref(const QString &path);