refactor(common): de-Qt oakcommon and wrap it in a pure C ABI

- de-Qt all classes under src/common (std::string/vector/mutex,
  std::filesystem, expat-based XmlStreamReader/Writer)
- add pure C ABI in include/common + src/common/c_api: opaque handles,
  init returns NULL on failure, free(NULL) is a no-op, out-params with
  negative OAKCOMMON_E_* error codes (include/common/error.h)
- remove single-consumer classes from oakcommon (html, jobtime,
  otioutils, playbackaudioclock, tohex, util, avframeptr,
  crashpadinterface/crashpadutils, autoscroll, digit, range); their
  destinations are recorded in docs/zh/plans/riir/notes.md
- add gtest suites under src/common/tests (127 cases), standalone
  build driver in src/common/standalone

include/common/ocioutils.h

@@ -0,0 +1,88 @@
+/***
+
+  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_OCIOUTILS_H
+#define OAK_EDITOR_OCIOUTILS_H
+
+#include "common/error.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Native pixel format codes, mirroring olive::core::PixelFormat
+ *
+ * The numeric values must stay in sync with
+ * olive/core/render/pixelformat.h (Format enum).
+ */
+enum OakCommonPixelFormat {
+	OAKCOMMON_PIXEL_FORMAT_INVALID = -1, /**< Invalid/unknown format. */
+	OAKCOMMON_PIXEL_FORMAT_U8 = 0, /**< 8-bit unsigned integer. */
+	OAKCOMMON_PIXEL_FORMAT_U10 = 1, /**< 10-bit unsigned integer. */
+	OAKCOMMON_PIXEL_FORMAT_U16 = 2, /**< 16-bit unsigned integer. */
+	OAKCOMMON_PIXEL_FORMAT_F16 = 3, /**< 16-bit float (half). */
+	OAKCOMMON_PIXEL_FORMAT_F32 = 4, /**< 32-bit float. */
+	OAKCOMMON_PIXEL_FORMAT_COUNT = 5 /**< Sentinel, not a valid format. */
+};
+
+/**
+ * @brief OpenColorIO bit depth codes, matching OCIO::BitDepth
+ *
+ * Returned through the out parameter of
+ * oakcommon_ocioutils_get_ocio_bit_depth_from_pixel_format() as a plain
+ * int so that callers never see OCIO types. Values match the OCIO
+ * BitDepth enum: 0 = unknown, 1 = uint8, 2 = uint10, 3 = uint12,
+ * 4 = uint14, 5 = uint16, 6 = uint32, 7 = f16, 8 = f32 (OCIO v2).
+ */
+typedef struct OakCommonOCIOUtils OakCommonOCIOUtils;
+
+/**
+ * @brief Creates an OCIOUtils handle
+ *
+ * The object is stateless; the handle exists only to satisfy the C API
+ * lifetime contract. Returns NULL on failure.
+ */
+OakCommonOCIOUtils *oakcommon_ocioutils_init(void);
+
+/**
+ * @brief Destroys an OCIOUtils handle; no-op on NULL
+ */
+void oakcommon_ocioutils_free(OakCommonOCIOUtils *self);
+
+/**
+ * @brief Maps a native pixel format to an OCIO bit depth
+ *
+ * @param self handle from oakcommon_ocioutils_init()
+ * @param pixel_format one of the OakCommonPixelFormat values
+ * @param out_bit_depth receives the OCIO bit depth as an int (see the
+ *        OakCommonOCIOUtils typedef documentation); set to 0
+ *        (BIT_DEPTH_UNKNOWN) for invalid formats
+ * @return OAKCOMMON_OK, or OAKCOMMON_E_INVALID if self or
+ *         out_bit_depth is NULL or pixel_format is not a known code
+ */
+int oakcommon_ocioutils_get_ocio_bit_depth_from_pixel_format(
+	OakCommonOCIOUtils *self, int pixel_format, int *out_bit_depth);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // OAK_EDITOR_OCIOUTILS_H