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/miscutils.h

@@ -0,0 +1,117 @@
+/***
+
+  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_MISCUTILS_H
+#define OAK_EDITOR_MISCUTILS_H
+
+#include "common/error.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Minimum decibel value used by the editor (-200.0 dB).
+ *
+ * In basically all circumstances, this calculates to 0.0 linear.
+ */
+#define OAKCOMMON_DECIBEL_MINIMUM (-200.0)
+
+/**
+ * @brief Convert a linear amplitude to decibels.
+ *
+ * A linear value of 0.0 (or anything yielding an infinite result) returns
+ * OAKCOMMON_DECIBEL_MINIMUM.
+ *
+ * @param linear Linear amplitude.
+ * @param out_db Receives the decibel value. Must not be NULL.
+ * @return OAKCOMMON_OK on success, OAKCOMMON_E_INVALID if out_db is NULL.
+ */
+int oakcommon_decibel_from_linear(double linear, double *out_db);
+
+/**
+ * @brief Convert decibels to a linear amplitude.
+ *
+ * Results below 1e-6 are clamped to 0.0.
+ *
+ * @param db Decibel value.
+ * @param out_linear Receives the linear amplitude. Must not be NULL.
+ * @return OAKCOMMON_OK on success, OAKCOMMON_E_INVALID if out_linear is NULL.
+ */
+int oakcommon_decibel_to_linear(double db, double *out_linear);
+
+/**
+ * @brief Convert a logarithmic slider position (0..1) to decibels.
+ *
+ * @param logarithmic Logarithmic position.
+ * @param out_db Receives the decibel value. Must not be NULL.
+ * @return OAKCOMMON_OK on success, OAKCOMMON_E_INVALID if out_db is NULL.
+ */
+int oakcommon_decibel_from_logarithmic(double logarithmic, double *out_db);
+
+/**
+ * @brief Convert decibels to a logarithmic slider position (0..1).
+ *
+ * @param db Decibel value.
+ * @param out_logarithmic Receives the logarithmic position. Must not be NULL.
+ * @return OAKCOMMON_OK on success, OAKCOMMON_E_INVALID if out_logarithmic
+ * is NULL.
+ */
+int oakcommon_decibel_to_logarithmic(double db, double *out_logarithmic);
+
+/**
+ * @brief Convert a linear amplitude directly to a logarithmic position.
+ *
+ * @param linear Linear amplitude.
+ * @param out_logarithmic Receives the logarithmic position. Must not be NULL.
+ * @return OAKCOMMON_OK on success, OAKCOMMON_E_INVALID if out_logarithmic
+ * is NULL.
+ */
+int oakcommon_decibel_linear_to_logarithmic(double linear,
+											double *out_logarithmic);
+
+/**
+ * @brief Convert a logarithmic position directly to a linear amplitude.
+ *
+ * @param logarithmic Logarithmic position.
+ * @param out_linear Receives the linear amplitude. Must not be NULL.
+ * @return OAKCOMMON_OK on success, OAKCOMMON_E_INVALID if out_linear is NULL.
+ */
+int oakcommon_decibel_logarithmic_to_linear(double logarithmic,
+											double *out_linear);
+
+/**
+ * @brief Linearly interpolate between a and b using t.
+ *
+ * t should be between 0.0 and 1.0: 0.0 returns a, 1.0 returns b.
+ *
+ * @param a Start value.
+ * @param b End value.
+ * @param t Interpolation factor.
+ * @param out_value Receives the interpolated value. Must not be NULL.
+ * @return OAKCOMMON_OK on success, OAKCOMMON_E_INVALID if out_value is NULL.
+ */
+int oakcommon_lerp(double a, double b, double t, double *out_value);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif //OAK_EDITOR_MISCUTILS_H