refactor(render): de-Qt oakrender and wrap it in a pure C ABI

- copy engine/render to src/render/src (sunk param types excluded),
  de-Qt in five parallel groups: core machinery (tickets/worker pool/
  jobs), caches, color/texture, preview/IPC, GPU backends
- replace Qt GL/Vulkan wrappers with native context abstractions
  (CGL/EGL/WGL, raw vulkan.h), QProcess with POSIX WorkerProcess,
  QJsonObject with a minimal NDJSON-compatible workerjson (wire
  protocol unchanged), QDataStream disk state with a byte-compatible
  BinaryStream
- signals become single std::function callbacks or facade-triggered
  calls per the documented signal/slot strategy
- pure C ABI in include/render + src/render/c_api (renderer/cache/
  color/manager families, OAKRENDER_E_* codes), 37 gtest cases
- bridge src/node/transition/render/* stubs to the real oakrender
  headers, closing the node<->render cycle: liboaknode links
  liboakrender, zero dangling symbols
- docs: M7 implementation status + oakrender semantic-change notes

src/render/tests/manager_test.cpp

@@ -0,0 +1,130 @@
+/***
+
+  Oak Video Editor - 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/>.
+
+***/
+
+#include "render/manager.h"
+
+#include <vector>
+
+#include <gtest/gtest.h>
+
+#include "node/factory.h"
+#include "node/node.h"
+
+namespace
+{
+
+void noop_frame_ready(OakCodecFrame *frame, int64_t ts, void *userdata)
+{
+	(void) frame;
+	(void) ts;
+	(void) userdata;
+}
+
+} // namespace
+
+/* ---- Manager lifecycle ----------------------------------------------------
+ *
+ * RenderManager's constructor reads the not-yet-split config module
+ * (OAK_CONFIG) and spawns backend/worker machinery whose symbols dangle
+ * in the standalone build (-undefined dynamic_lookup), so init/shutdown
+ * and everything requiring a live manager cannot run here (M7 §4). The
+ * no-manager error paths below are the runnable half.
+ */
+
+TEST(OakRenderManagerTest, InitShutdown)
+{
+	GTEST_SKIP() << "RenderManager pulls dangling config/codec/worker "
+					"symbols in the standalone build (M7 §4)";
+}
+
+TEST(OakRenderManagerTest, RequestFrameRequiresManager)
+{
+	ASSERT_EQ(oaknode_factory_initialize(), OAKNODE_OK);
+	OakNodeNode *viewer = oaknode_factory_create_from_id(
+		"org.olivevideoeditor.Olive.vieweroutput");
+	ASSERT_NE(viewer, nullptr);
+
+	// No oakrender_manager_init() in this process: E_STATE
+	EXPECT_EQ(oakrender_request_frame(viewer, 0, noop_frame_ready, nullptr),
+			  int64_t(OAKRENDER_E_STATE));
+
+	oaknode_node_free(viewer);
+	oaknode_factory_destroy();
+}
+
+TEST(OakRenderManagerTest, RequestFrameInvalidArgs)
+{
+	EXPECT_EQ(oakrender_request_frame(nullptr, 0, noop_frame_ready, nullptr),
+			  int64_t(OAKRENDER_E_INVALID));
+
+	ASSERT_EQ(oaknode_factory_initialize(), OAKNODE_OK);
+	OakNodeNode *viewer = oaknode_factory_create_from_id(
+		"org.olivevideoeditor.Olive.vieweroutput");
+	ASSERT_NE(viewer, nullptr);
+	EXPECT_EQ(oakrender_request_frame(viewer, 0, nullptr, nullptr),
+			  int64_t(OAKRENDER_E_INVALID));
+	oaknode_node_free(viewer);
+	oaknode_factory_destroy();
+}
+
+TEST(OakRenderManagerTest, RequestFrameWithManager)
+{
+	GTEST_SKIP() << "A live frame request needs the render manager plus "
+					"codec/worker symbols that dangle standalone (M7 §4)";
+}
+
+TEST(OakRenderManagerTest, CancelUnknownRequest)
+{
+	EXPECT_EQ(oakrender_cancel_request(-1), OAKRENDER_E_NOT_FOUND);
+	EXPECT_EQ(oakrender_cancel_request(424242), OAKRENDER_E_NOT_FOUND);
+}
+
+TEST(OakRenderManagerTest, CacherSettersRequireManager)
+{
+	EXPECT_EQ(oakrender_set_cacher_multicam(nullptr), OAKRENDER_E_STATE);
+	EXPECT_EQ(oakrender_set_display_color_processor(nullptr),
+			  OAKRENDER_E_STATE);
+}
+
+/* ---- Disk cache ----------------------------------------------------------- */
+
+TEST(OakRenderDiskCacheTest, PathTwoStage)
+{
+	const int required = oakrender_disk_cache_path(nullptr, 0);
+	ASSERT_GT(required, 1);
+
+	std::vector<char> buf(static_cast<size_t>(required));
+	EXPECT_EQ(oakrender_disk_cache_path(buf.data(), required), required);
+	EXPECT_STRNE(buf.data(), "");
+
+	// Too-small buffer: size query only, no write
+	std::vector<char> small(2, 0);
+	EXPECT_EQ(oakrender_disk_cache_path(small.data(), 2), required);
+}
+
+TEST(OakRenderDiskCacheTest, SizeAndClear)
+{
+	// Lazily creates the DiskManager singleton; consumption is
+	// non-negative by definition.
+	EXPECT_GE(oakrender_disk_cache_size(), int64_t(0));
+
+	// Clearing an existing (possibly empty) default folder succeeds.
+	EXPECT_EQ(oakrender_disk_cache_clear(), OAKRENDER_OK);
+}