audio: master-clock playback timing, output clock compensation, buffer config, interpolated speed

- playback timer uses the audio output device as its master clock: the
  PortAudio callback counts consumed frames (including underrun
  zero-fill) so video cannot drift away from what is heard; wall clock
  remains as fallback when no clocked output is running
- output clock compensates for device output latency; new Preferences >
  Audio buffer size setting (0 = auto)
- SampleBuffer::speed() now uses linear interpolation instead of
  nearest-neighbor sampling
- regression tests: audio-clock driven timer (fwd/rev/speed), wall
  clock fallback, interpolation correctness

app/common/CMakeLists.txt

@@ -43,6 +43,7 @@ target_sources(libolive-editor PRIVATE
         oiioutils.cpp
         oiioutils.h
         otioutils.h
+        playbackaudioclock.h
         qtutils.cpp
         qtutils.h
         range.h

app/common/playbackaudioclock.h

@@ -0,0 +1,45 @@
+/***
+
+  Oak - 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_PLAYBACKAUDIOCLOCK_H
+#define OAK_PLAYBACKAUDIOCLOCK_H
+
+namespace olive
+{
+
+/**
+ * @brief Source of an audio output clock for playback timing
+ */
+class PlaybackAudioClock {
+public:
+	virtual ~PlaybackAudioClock() = default;
+
+	/**
+	 * @brief Seconds of audio consumed by the output device
+	 *
+	 * Must return a negative value when no clocked output is running, in
+	 * which case the caller should fall back to the wall clock.
+	 */
+	virtual double seconds() const = 0;
+};
+
+}
+
+#endif // OAK_PLAYBACKAUDIOCLOCK_H