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/debug.h

@@ -0,0 +1,75 @@
+/***
+
+  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_DEBUG_H
+#define OAK_EDITOR_DEBUG_H
+
+#include "common/error.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Severity levels for oakcommon debug output.
+ *
+ * Mirrors olive::DebugLevel in src/common/src/debug.h.
+ */
+enum OakCommonDebugLevel {
+	OAKCOMMON_DEBUG_DEBUG = 0, /**< Verbose debug message. */
+	OAKCOMMON_DEBUG_INFO = 1, /**< Informational message. */
+	OAKCOMMON_DEBUG_WARNING = 2, /**< Warning message. */
+	OAKCOMMON_DEBUG_ERROR = 3, /**< Error message. */
+	OAKCOMMON_DEBUG_FATAL = 4 /**< Fatal error message. */
+};
+
+/**
+ * @brief Print a debug message to stderr, prefixed with its level.
+ *
+ * De-Qt replacement for the old Qt message handler. The line is
+ * flushed immediately.
+ *
+ * @param level One of OakCommonDebugLevel; out-of-range values print
+ * as "UNKNOWN".
+ * @param msg NUL-terminated message; NULL is treated as an empty
+ * string.
+ * @return OAKCOMMON_OK on success, OAKCOMMON_E_INVALID if msg is NULL.
+ */
+int oakcommon_debug_log(int level, const char *msg);
+
+/**
+ * @brief Copy the printable name of a debug level into buf.
+ *
+ * Two-segment string getter: if buf is NULL or buf_size is too small,
+ * nothing is written.
+ *
+ * @param level One of OakCommonDebugLevel.
+ * @param buf Destination buffer, may be NULL to query the size.
+ * @param buf_size Size of buf in bytes.
+ * @return Required buffer size in bytes including the terminating NUL
+ * (non-negative).
+ */
+int oakcommon_debug_level_name(int level, char *buf, int buf_size);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // OAK_EDITOR_DEBUG_H