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/error.h

@@ -0,0 +1,41 @@
+/***
+
+  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_ERROR_H
+#define OAK_EDITOR_ERROR_H
+
+/**
+ * @brief Status and error codes shared by all oakcommon C API families.
+ *
+ * Return-code convention (mirrors engine/include/oakengine/init.h):
+ * 0 (OAKCOMMON_OK) on success, a negative OAKCOMMON_E_* error code on
+ * failure. String getters return the required buffer size in bytes
+ * (including the terminating NUL) as a non-negative value instead.
+ */
+#define OAKCOMMON_OK 0 /**< Success. */
+#define OAKCOMMON_E_INVALID (-1) /**< NULL handle or invalid argument. */
+#define OAKCOMMON_E_STATE (-2) /**< Call not valid in the current state. */
+#define OAKCOMMON_E_FAILED (-3) /**< The underlying operation failed. */
+#define OAKCOMMON_E_NOT_FOUND (-4) /**< Index out of range / entry not found. */
+#define OAKCOMMON_E_NOMEM (-5) /**< Allocation failed. */
+
+#define SUCCESS OAKCOMMON_OK /**< @deprecated Use OAKCOMMON_OK. */
+
+#endif //OAK_EDITOR_ERROR_H