refactor(codec): de-Qt oakcodec and wrap it in a pure C ABI; switch common handles to refcounted value structs

- oakcodec: de-Qt all 20 sources (QThread decode loop -> std::thread,
  QObject/signals -> callbacks), pure C ABI in include/codec with
  refcounted neutral handles (OakFrame/OakDecoder/OakEncoder),
  framemanager moved in from render, frame_to_buffer/buffer_to_frame
  moved in from oakcommon oiioutils, codec->task via submit callback
  (M8 will register), all cross-module calls go through the other
  side's C API, -fvisibility=hidden + OAKCODEC_API
- oakcommon: handles become refcounted value structs
  {ctx, addref, release, abi_version} (FFmpeg-style), pass-by-value
  signatures, free() as release wrapper; init_from_native/get_native
  for copyable value objects; OakCommonXxx renamed to OakXxx
- oakcommon: add logging (log_debug/info/warning/critical with level
  filtering and sink injection) + printf-style oakcommon_log C wrapper
- oakrender: add CancelAtom C API family; complete
  oakrender_color_processor_convert_frame; fix get_processor() missing
  definition and OCIO env var lookup
- tests: oakcommon 174, oaknode 96, oakrender 42, oakcodec 18, all
  green in their standalone builds

include/common/handle.h

@@ -0,0 +1,69 @@
+/***
+
+  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_HANDLE_H
+#define OAK_EDITOR_HANDLE_H
+
+#include <stdint.h>
+
+/**
+ * @brief Current ABI version stamped into every oakcommon handle.
+ *
+ * Bump whenever the handle layout or the semantics of any exported
+ * function change incompatibly. Consumers should compare a handle's
+ * abi_version field against the value they were compiled with before
+ * dereferencing ctx.
+ */
+#define OAKCOMMON_ABI_VERSION 1
+
+/**
+ * @brief Neutral handle convention shared by all oakcommon wrappers.
+ *
+ * Every wrapper type is a by-value struct with the same four fields:
+ *
+ *     typedef struct OakXxx {
+ *         void *ctx;                       // opaque, points to the impl
+ *         void (*addref)(void *ctx);       // atomic +1, owner-DLL code
+ *         void (*release)(void *ctx);      // atomic -1, destroys at 0
+ *         uint32_t abi_version;            // OAKCOMMON_ABI_VERSION
+ *     } OakXxx;
+ *
+ * Rules:
+ * - oakcommon_<name>_init*() returns a handle whose underlying object
+ *   has reference count 1.
+ * - Copying the struct copies the pointer, not the count: call
+ *   handle.addref(handle.ctx) for every additional long-lived copy and
+ *   handle.release(handle.ctx) (or the oakcommon_<name>_free()
+ *   convenience wrapper) when done with each copy.
+ * - release() decrements the atomic count and destroys the underlying
+ *   object when it reaches zero; the destructor runs in the DLL that
+ *   created the object, so cross-DLL handing is safe.
+ * - The struct itself carries no ownership: it is never heap-allocated
+ *   by the API, so it needs no destruction of its own.
+ * - Functions that only read a handle take it BY VALUE (OakXxx self);
+ *   an empty handle (ctx == NULL) is reported as OAKCOMMON_E_INVALID.
+ *   oakcommon_<name>_free() deliberately stays a pointer API
+ *   (OakXxx *h, like av_frame_unref()/av_buffer_unref()) so it can
+ *   null out the caller's ctx after the final release; NULL and
+ *   ctx == NULL are no-ops. Out parameters that produce a handle
+ *   (e.g. option/positional-argument registration) also stay pointers.
+ */
+
+#endif //OAK_EDITOR_HANDLE_H