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/dropworkflowbehavior.cpp

@@ -0,0 +1,66 @@
+/***
+
+  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/dropworkflowbehavior.h"
+
+#include <cstring>
+
+#include "../src/dropworkflowbehavior.h"
+
+static const char *behavior_name(int value)
+{
+	switch (value) {
+	case olive::k_dws_ask:
+		return "ASK";
+	case olive::k_dws_auto:
+		return "AUTO";
+	case olive::k_dws_manual:
+		return "MANUAL";
+	case olive::k_dws_disable:
+		return "DISABLE";
+	}
+	return "UNKNOWN";
+}
+
+int oakcommon_drop_workflow_behavior_is_valid(int value)
+{
+	switch (value) {
+	case olive::k_dws_ask:
+	case olive::k_dws_auto:
+	case olive::k_dws_manual:
+	case olive::k_dws_disable:
+		return 1;
+	}
+	return 0;
+}
+
+int oakcommon_drop_workflow_behavior_name(int value, char *buf, int buf_size)
+{
+	try {
+		const char *name = behavior_name(value);
+		int needed = (int)strlen(name) + 1;
+
+		if (buf && buf_size >= needed)
+			memcpy(buf, name, needed);
+		return needed;
+	} catch (...) {
+		return OAKCOMMON_E_FAILED;
+	}
+}