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/power.h

@@ -0,0 +1,58 @@
+/***
+
+  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_POWER_H
+#define OAK_EDITOR_POWER_H
+
+#include <stdint.h>
+
+#include "error.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Round `value` up to the next power of two
+ *
+ * Stateless pure function, no handle required. Writes the result to `out`.
+ *
+ * @param value Input value.
+ * @param out Receives the rounded value. Must not be NULL.
+ * @return OAKCOMMON_OK on success, OAKCOMMON_E_INVALID if `out` is NULL.
+ */
+int oakcommon_power_ceil_to_power_of_2(uint32_t value, uint32_t *out);
+
+/**
+ * @brief Round `value` down to the nearest power of two
+ *
+ * Stateless pure function, no handle required. Writes the result to `out`.
+ *
+ * @param value Input value.
+ * @param out Receives the rounded value. Must not be NULL.
+ * @return OAKCOMMON_OK on success, OAKCOMMON_E_INVALID if `out` is NULL.
+ */
+int oakcommon_power_floor_to_power_of_2(uint32_t value, uint32_t *out);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif //OAK_EDITOR_POWER_H