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

include/render/cache.h

@@ -0,0 +1,146 @@
+/***
+
+  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/>.
+
+***/
+
+#ifndef OAK_EDITOR_RENDER_CACHE_H
+#define OAK_EDITOR_RENDER_CACHE_H
+
+#include <stdint.h>
+
+// Same-dir quoted includes: inside this build the engine-style spelling
+// "render/renderer.h" resolves to the transition bridge headers, so the
+// public headers reference each other relative to their own directory.
+#include "error.h"
+#include "renderer.h" /* OakCodecFrame */
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @file cache.h
+ * @brief C ABI for the oakrender playback/frame-hash caches
+ *        (olive::PlaybackCache / olive::FrameHashCache), M7 §2.2.
+ *
+ * An OakRenderCache IS a reinterpreted olive::FrameHashCache (created
+ * without a parent node), no wrapper allocation. Handles from
+ * oakrender_cache_create() are owned by the caller and must be released
+ * with oakrender_cache_free().
+ *
+ * All timestamps are int64 frame numbers in the cache's timebase (see
+ * oakrender_cache_set_timebase()); a cache without a valid timebase
+ * treats timestamps as whole seconds.
+ *
+ * No cache events cross the boundary (M7 §2.2, 2026-08 revision):
+ * invalidate/validate are triggered by and known to the caller; the
+ * facade re-emits notifications after the triggering command.
+ */
+typedef struct OakRenderCache OakRenderCache;
+
+/**
+ * @brief Create a detached frame hash cache (no parent node, no
+ * timebase). Owned by the caller.
+ *
+ * @return Cache handle, or NULL on allocation failure.
+ */
+OakRenderCache *oakrender_cache_create(void);
+
+/** @brief Destroy a cache created by oakrender_cache_create(). NULL-safe. */
+void oakrender_cache_free(OakRenderCache *cache);
+
+/**
+ * @brief Set the frame timebase used to interpret all timestamps of this
+ * cache (FrameHashCache::set_timebase()).
+ *
+ * @return OAKRENDER_OK, or OAKRENDER_E_INVALID for NULL cache or
+ *         non-positive num/den.
+ */
+int oakrender_cache_set_timebase(OakRenderCache *cache, int num, int den);
+
+/**
+ * @brief Set the cache UUID used in on-disk frame cache filenames
+ * (PlaybackCache::set_uuid()).
+ *
+ * @return OAKRENDER_OK or OAKRENDER_E_INVALID.
+ */
+int oakrender_cache_set_uuid(OakRenderCache *cache, const char *uuid);
+
+/**
+ * @brief Mark the timestamp range [in_ts, out_ts) invalidated
+ * (PlaybackCache::invalidate()). NULL cache is a no-op.
+ */
+void oakrender_cache_invalidate(OakRenderCache *cache, int64_t in_ts,
+								int64_t out_ts);
+
+/**
+ * @brief Mark the timestamp range [in_ts, out_ts) validated
+ * (PlaybackCache::validate()). NULL cache is a no-op.
+ */
+void oakrender_cache_validate(OakRenderCache *cache, int64_t in_ts,
+							  int64_t out_ts);
+
+/**
+ * @brief 1 when the cache holds any validated range
+ * (PlaybackCache::has_validated_ranges()), 0 otherwise / on NULL.
+ */
+int oakrender_cache_has_validated_ranges(const OakRenderCache *cache);
+
+/**
+ * @brief Timeline cache indicator height in pixels
+ * (PlaybackCache::get_cache_indicator_height()). Constant query.
+ */
+int oakrender_cache_indicator_height(void);
+
+/**
+ * @brief Load a cached frame from disk
+ * (FrameHashCache::load_cache_frame(cache_path, uuid, ts)).
+ *
+ * @param path Cache directory (e.g. oakrender_disk_cache_path()).
+ * @param uuid Cache UUID of the producing node.
+ * @param out_frame Receives an owned frame handle (release with
+ *        oakrender_codec_frame_free()).
+ *
+ * @return OAKRENDER_OK, OAKRENDER_E_INVALID (NULL argument), or
+ *         OAKRENDER_E_NOT_FOUND (no cached frame at `ts` / undecodable).
+ */
+int oakrender_frame_cache_load(OakRenderCache *cache, const char *path,
+							   const char *uuid, int64_t ts,
+							   OakCodecFrame **out_frame);
+
+/**
+ * @brief Save a frame to the disk cache under the cache's timebase and
+ * the frame's own timestamp (FrameHashCache::save_cache_frame()).
+ * NULL arguments are a no-op.
+ */
+void oakrender_frame_cache_save(OakRenderCache *cache, const char *path,
+								const char *uuid, const OakCodecFrame *frame);
+
+/* ---- Debug --------------------------------------------------------------- */
+
+/**
+ * @brief Number of live oakrender-owned objects (caches, textures,
+ * frames, color processors) for leak assertions in tests.
+ */
+int oakrender_debug_alive_count(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif //OAK_EDITOR_RENDER_CACHE_H