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/xmlutils.h

@@ -0,0 +1,156 @@
+/***
+
+  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_XMLUTILS_H
+#define OAK_EDITOR_XMLUTILS_H
+
+#include "common/error.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct OakCommonXmlReader OakCommonXmlReader;
+typedef struct OakCommonXmlWriter OakCommonXmlWriter;
+
+/**
+ * @brief Create a streaming XML reader over a complete document.
+ *
+ * @param data NUL-terminated XML text. Must not be NULL.
+ * @return A new reader, or NULL on failure (NULL data, out of memory).
+ */
+OakCommonXmlReader *oakcommon_xml_reader_init(const char *data);
+
+/**
+ * @brief Destroy a reader. No-op on NULL.
+ */
+void oakcommon_xml_reader_free(OakCommonXmlReader *reader);
+
+/**
+ * @brief Advance until the next start element, an end element, or the end
+ *        of the document.
+ *
+ * @param reader Reader handle.
+ * @param found Out: 1 if positioned on a start element, 0 otherwise.
+ * @return OAKCOMMON_OK or a negative OAKCOMMON_E_* error code.
+ */
+int oakcommon_xml_reader_read_next_start_element(OakCommonXmlReader *reader,
+												 int *found);
+
+/**
+ * @brief Name of the current element token.
+ *
+ * @return Required buffer size in bytes (including NUL), or a negative
+ *         OAKCOMMON_E_* error code.
+ */
+int oakcommon_xml_reader_name(OakCommonXmlReader *reader, char *buf,
+							  int buf_size);
+
+/**
+ * @brief Read the concatenated character data of the current element.
+ *
+ * Must be called on a start element; consumes up to the matching end
+ * element.
+ *
+ * @return Required buffer size in bytes (including NUL), or a negative
+ *         OAKCOMMON_E_* error code.
+ */
+int oakcommon_xml_reader_read_element_text(OakCommonXmlReader *reader,
+										   char *buf, int buf_size);
+
+/**
+ * @brief Skip the current element and all of its children.
+ *
+ * @return OAKCOMMON_OK or a negative OAKCOMMON_E_* error code.
+ */
+int oakcommon_xml_reader_skip_current_element(OakCommonXmlReader *reader);
+
+/**
+ * @brief Number of attributes on the current start element.
+ *
+ * @return OAKCOMMON_OK or a negative OAKCOMMON_E_* error code.
+ */
+int oakcommon_xml_reader_attribute_count(OakCommonXmlReader *reader,
+										 int *count);
+
+/**
+ * @brief Name of the attribute at @p index on the current start element.
+ *
+ * @return Required buffer size in bytes (including NUL), OAKCOMMON_E_NOT_FOUND
+ *         if @p index is out of range, or another negative OAKCOMMON_E_* code.
+ */
+int oakcommon_xml_reader_attribute_name(OakCommonXmlReader *reader, int index,
+										char *buf, int buf_size);
+
+/**
+ * @brief Value of the attribute at @p index on the current start element.
+ *
+ * @return Required buffer size in bytes (including NUL), OAKCOMMON_E_NOT_FOUND
+ *         if @p index is out of range, or another negative OAKCOMMON_E_* code.
+ */
+int oakcommon_xml_reader_attribute_value(OakCommonXmlReader *reader,
+										 int index, char *buf, int buf_size);
+
+/**
+ * @brief Whether the document failed to parse.
+ *
+ * @return OAKCOMMON_OK or a negative OAKCOMMON_E_* error code.
+ */
+int oakcommon_xml_reader_has_error(OakCommonXmlReader *reader,
+								   int *has_error);
+
+/**
+ * @brief Create a streaming XML writer.
+ *
+ * @return A new writer, or NULL on failure.
+ */
+OakCommonXmlWriter *oakcommon_xml_writer_init(void);
+
+/**
+ * @brief Destroy a writer. No-op on NULL.
+ */
+void oakcommon_xml_writer_free(OakCommonXmlWriter *writer);
+
+int oakcommon_xml_writer_write_start_element(OakCommonXmlWriter *writer,
+											 const char *name);
+int oakcommon_xml_writer_write_attribute(OakCommonXmlWriter *writer,
+										 const char *name, const char *value);
+int oakcommon_xml_writer_write_characters(OakCommonXmlWriter *writer,
+										  const char *text);
+int oakcommon_xml_writer_write_text_element(OakCommonXmlWriter *writer,
+											const char *name,
+											const char *text);
+int oakcommon_xml_writer_write_end_element(OakCommonXmlWriter *writer);
+int oakcommon_xml_writer_write_end_document(OakCommonXmlWriter *writer);
+
+/**
+ * @brief The document written so far.
+ *
+ * @return Required buffer size in bytes (including NUL), or a negative
+ *         OAKCOMMON_E_* error code.
+ */
+int oakcommon_xml_writer_output(OakCommonXmlWriter *writer, char *buf,
+								int buf_size);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // OAK_EDITOR_XMLUTILS_H