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/ffmpegutils.h

@@ -0,0 +1,116 @@
+/***
+
+  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_FFMPEGUTILS_H
+#define OAK_EDITOR_FFMPEGUTILS_H
+
+#include "common/error.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Stateless mappings between native pixel/sample formats and the
+ * opaque FBPixelFormat / FBSampleFormat constants of ffmpeg_bridge
+ *
+ * All functions are pure format conversions; there is no handle to create
+ * or free. Native pixel/sample formats are passed as plain ints matching
+ * the olive::core::PixelFormat::Format / SampleFormat::Format enum values
+ * (invalid = -1). Bridge formats are the fb_pix_fmt_* / fb_sample_fmt_*
+ * constants from ffmpeg_bridge/ffmpeg_bridge.h.
+ */
+
+/**
+ * @brief RGB / RGBA channel counts (flattened from VideoParams)
+ */
+#define OAKCOMMON_RGB_CHANNEL_COUNT 3
+#define OAKCOMMON_RGBA_CHANNEL_COUNT 4
+
+/**
+ * @brief Returns a bridge pixel format that a frame can be converted to
+ * with minimal data loss, clamped to a maximum native precision
+ *
+ * @param pix_fmt bridge pixel format to find a compatible conversion for
+ * @param maximum_pix_fmt maximum native pixel format, or -1 for no limit
+ * @param out receives the chosen bridge pixel format
+ * @return OAKCOMMON_OK on success, OAKCOMMON_E_INVALID if out is NULL
+ */
+int oakcommon_ffmpegutils_get_compatible_bridge_pixel_format(
+	int pix_fmt, int maximum_pix_fmt, int *out);
+
+/**
+ * @brief Returns a native pixel format usable to convert from a native
+ * frame to a bridge frame with minimal data loss
+ *
+ * @param pix_fmt native pixel format
+ * @param out receives the compatible native pixel format (-1 if none)
+ * @return OAKCOMMON_OK on success, OAKCOMMON_E_INVALID if out is NULL
+ */
+int oakcommon_ffmpegutils_get_compatible_pixel_format(int pix_fmt,
+						      int *out);
+
+/**
+ * @brief Returns a bridge pixel format for a given native pixel format
+ *
+ * @param pix_fmt native pixel format
+ * @param channel_count OAKCOMMON_RGB_CHANNEL_COUNT or
+ * OAKCOMMON_RGBA_CHANNEL_COUNT
+ * @param out receives the bridge pixel format (fb_pix_fmt_none if none)
+ * @return OAKCOMMON_OK on success, OAKCOMMON_E_INVALID if out is NULL
+ */
+int oakcommon_ffmpegutils_get_ffmpeg_pixel_format(int pix_fmt,
+						  int channel_count,
+						  int *out);
+
+/**
+ * @brief Returns a native sample format for a given bridge sample format
+ *
+ * @param smp_fmt bridge sample format
+ * @param out receives the native sample format (-1 if unknown)
+ * @return OAKCOMMON_OK on success, OAKCOMMON_E_INVALID if out is NULL
+ */
+int oakcommon_ffmpegutils_get_native_sample_format(int smp_fmt, int *out);
+
+/**
+ * @brief Returns a bridge sample format for a given native sample format
+ *
+ * @param smp_fmt native sample format
+ * @param out receives the bridge sample format (fb_sample_fmt_none if none)
+ * @return OAKCOMMON_OK on success, OAKCOMMON_E_INVALID if out is NULL
+ */
+int oakcommon_ffmpegutils_get_ffmpeg_sample_format(int smp_fmt, int *out);
+
+/**
+ * @brief Converts a "JPEG" full-range bridge pixel format to its regular
+ * counterpart
+ *
+ * @param pix_fmt bridge pixel format
+ * @param out receives the regular-range format (unchanged if not JPEG)
+ * @return OAKCOMMON_OK on success, OAKCOMMON_E_INVALID if out is NULL
+ */
+int oakcommon_ffmpegutils_convert_jpeg_space_to_regular_space(int pix_fmt,
+							      int *out);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif //OAK_EDITOR_FFMPEGUTILS_H