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

src/common/c_api/ocioutils.cpp

@@ -0,0 +1,62 @@
+/***
+
+  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 "common/ocioutils.h"
+
+#include <new>
+
+#include "../src/ocioutils.h"
+
+struct OakCommonOCIOUtils {
+	int unused; /**< Stateless; only the address matters. */
+};
+
+OakCommonOCIOUtils *oakcommon_ocioutils_init(void)
+{
+	try {
+		return new (std::nothrow) OakCommonOCIOUtils{};
+	} catch (...) {
+		return NULL;
+	}
+}
+
+void oakcommon_ocioutils_free(OakCommonOCIOUtils *self)
+{
+	delete self;
+}
+
+int oakcommon_ocioutils_get_ocio_bit_depth_from_pixel_format(
+	OakCommonOCIOUtils *self, int pixel_format, int *out_bit_depth)
+{
+	try {
+		if (self == NULL || out_bit_depth == NULL)
+			return OAKCOMMON_E_INVALID;
+		if (pixel_format < OAKCOMMON_PIXEL_FORMAT_INVALID ||
+			pixel_format >= OAKCOMMON_PIXEL_FORMAT_COUNT)
+			return OAKCOMMON_E_INVALID;
+
+		ocio::BitDepth depth = OCIOUtils::get_ocio_bit_depth_from_pixel_format(
+			static_cast<olive::core::PixelFormat::Format>(pixel_format));
+		*out_bit_depth = static_cast<int>(depth);
+		return OAKCOMMON_OK;
+	} catch (...) {
+		return OAKCOMMON_E_FAILED;
+	}
+}