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/dropworkflowbehavior.h

@@ -0,0 +1,73 @@
+/***
+
+  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_DROPWORKFLOWBEHAVIOR_H
+#define OAK_EDITOR_DROPWORKFLOWBEHAVIOR_H
+
+#include "common/error.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Behavior when media is dropped onto a timeline without a
+ * sequence.
+ *
+ * Mirrors olive::DropWithoutSequenceBehavior in
+ * src/common/src/dropworkflowbehavior.h; enumerator order and values
+ * must stay identical because the config layer persists them as ints.
+ */
+enum OakCommonDropWorkflowBehavior {
+	OAKCOMMON_DWS_ASK = 0, /**< Ask the user every time. */
+	OAKCOMMON_DWS_AUTO = 1, /**< Automatically create a sequence. */
+	OAKCOMMON_DWS_MANUAL = 2, /**< Never create; import manually. */
+	OAKCOMMON_DWS_DISABLE = 3 /**< Disable dropping entirely. */
+};
+
+/**
+ * @brief Check whether value is a valid OakCommonDropWorkflowBehavior.
+ *
+ * @param value Integer behavior value (e.g. read from config).
+ * @return 1 if valid, 0 otherwise (this is a predicate, not a status
+ * code).
+ */
+int oakcommon_drop_workflow_behavior_is_valid(int value);
+
+/**
+ * @brief Copy the printable name of a behavior into buf.
+ *
+ * Two-segment string getter: if buf is NULL or buf_size is too small,
+ * nothing is written. Invalid values yield "UNKNOWN".
+ *
+ * @param value One of OakCommonDropWorkflowBehavior.
+ * @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_drop_workflow_behavior_name(int value, char *buf,
+					  int buf_size);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // OAK_EDITOR_DROPWORKFLOWBEHAVIOR_H