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)

worker/workermain.cpp

@@ -515,8 +515,12 @@ private:
 											static_cast<void *>(nullptr)));
 		ticket->setProperty(
 			"ipc_input_pool",
+			// The engine reads this back as the internal implementation object
+			// (olive::engine::internal::ipc::FrameSlotPool), which is exactly
+			// what the C handle points at.
 			olive::QtUtils::ptr_to_value(input_pool_ ?
-										   static_cast<void *>(&*input_pool_) :
+										   static_cast<void *>(
+											   input_pool_->handle()) :
 										   static_cast<void *>(nullptr)));
 		QVariantList input_slot_values;
 		for (int slot : input_slots) {