feat(app): multicam panel with live angle grid, switching, timeline enable

- New MulticamPanel: rows/cols angle grid with the current angle
  highlighted, click-to-switch, 1-9 switch-and-split and cmd-1-9
  switch-only shortcuts (focused-panel routed), deferred switch queue
  during playback.
- src/oakui/multicam.rs: clip->connected-sequence resolution, multicam
  state detection (selection then playhead fallbacks), per-angle frame
  requests rendered through the process backend into an LRU cache.
- Timeline clip context menu Multi-Cam checkable item wired to
  oaktimeline::multicam enable/disable with undo.
- Engine trait extended (real + mock); mock drives the real command
  path with synthesized angle frames.

crates/oakengine.bk/include/audio/levelmeter.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_AUDIO_LEVELMETER_H
+#define OAK_EDITOR_AUDIO_LEVELMETER_H
+
+#include "error.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @file levelmeter.h
+ * @brief C ABI for the oakaudio level meter (olive::AudioLevelMeter):
+ *        stateless peak/RMS/VU/LUFS analysis of planar float audio.
+ */
+
+/** Per-channel analysis results. dB fields floor at -200. */
+typedef struct oakaudio_channel_stats {
+	double peak_linear;
+	double peak_db;
+	double rms_linear;
+	double rms_db;
+	double vu_db;
+} oakaudio_channel_stats;
+
+/** Buffer-wide summary. */
+typedef struct oakaudio_meter_stats {
+	double max_peak_linear;
+	double integrated_lufs; /**< BS.1770-compatible unit (no K-weighting). */
+	int silence; /**< 1 when the buffer is (near-)silent. */
+} oakaudio_meter_stats;
+
+/**
+ * @brief Analyze a planar float buffer.
+ *
+ * @param planar Per-channel float planes.
+ * @param channel_count Number of channels (> 0).
+ * @param frame_count Frames per channel (>= 0).
+ * @param channels Receives per-channel stats; may be NULL.
+ * @param channels_capacity Capacity of `channels` (must be >=
+ *        channel_count when channels is non-NULL).
+ * @param summary Receives the buffer-wide summary; may be NULL.
+ * @return OAKAUDIO_OK or OAKAUDIO_E_INVALID.
+ */
+OAKAUDIO_API int oakaudio_levelmeter_analyze(const float *const *planar,
+		int channel_count, int frame_count,
+		oakaudio_channel_stats *channels, int channels_capacity,
+		oakaudio_meter_stats *summary);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif //OAK_EDITOR_AUDIO_LEVELMETER_H