build: split the engine into liboakengine.so; worker drops the UI entirely

Physical split: app/{audio,cli,codec,common,config,node,pluginSupport,
render,task,timeline,undo,tool,shaders} plus coreengine, version and
ui/icons+colorcoding move to a new top-level engine/ tree, built as
liboakengine.so (shared). The render backends (oakgl/oakvulkan) move
with it and link the engine library instead of embedding a static
render-core subset (libolive-rendercore is gone).

- oak-render-worker now links liboakengine instead of the whole
  libolive-editor object set: 336MB -> 2.9MB, no Qt Widgets UI
- the editor links liboakengine for the engine and keeps only UI
  objects in libolive-editor
- install/packaging: GNUInstallDirs libdir on Linux, bundle copy on
  macOS, oakengine.dll staged for NSIS, AppImage validation entry
- fix backend lookup for the new layout: DynamicRenderer searched
  ../app but backends now live in engine/; a stale pre-split liboakgl
  in the build tree got dlopened instead, re-initialized and later
  destroyed the interposed engine statics (full-suite segfault at
  DialogSequenceParameterTab, found via gdb watchpoint)

engine/audio/audiolevelmeter.cpp

@@ -0,0 +1,104 @@
+/***
+
+  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/>.
+
+***/
+
+#include "audiolevelmeter.h"
+
+#include <algorithm>
+#include <cmath>
+
+#include "common/decibel.h"
+
+namespace olive
+{
+
+AudioLevelMeter::Stats
+AudioLevelMeter::analyze_sample_buffer(const core::SampleBuffer &samples)
+{
+	Stats stats;
+
+	const int channel_count = samples.channel_count();
+	const size_t sample_count = samples.sample_count();
+	stats.channels.resize(channel_count);
+
+	if (!channel_count || !sample_count) {
+		return stats;
+	}
+
+	double total_square = 0.0;
+	size_t total_samples = 0;
+
+	for (int channel = 0; channel < channel_count; channel++) {
+		const float *channel_data = samples.data(channel);
+		double peak = 0.0;
+		double square_sum = 0.0;
+
+		for (size_t sample = 0; sample < sample_count; sample++) {
+			const double value = channel_data[sample];
+			const double abs_value = std::abs(value);
+
+			peak = std::max(peak, abs_value);
+			square_sum += value * value;
+		}
+
+		const double mean_square =
+			square_sum / static_cast<double>(sample_count);
+		const double rms = std::sqrt(mean_square);
+
+		ChannelStats channel_stats;
+		channel_stats.peak_linear = peak;
+		channel_stats.peak_db = linear_to_db(peak);
+		channel_stats.rms_linear = rms;
+		channel_stats.rms_db = linear_to_db(rms);
+		channel_stats.vu_db = channel_stats.rms_db;
+		stats.channels[channel] = channel_stats;
+
+		stats.max_peak_linear = std::max(stats.max_peak_linear, peak);
+		total_square += square_sum;
+		total_samples += sample_count;
+	}
+
+	stats.silence = qFuzzyIsNull(stats.max_peak_linear);
+	stats.integrated_lufs =
+		power_to_lufs(total_square / static_cast<double>(total_samples));
+
+	return stats;
+}
+
+double AudioLevelMeter::linear_to_db(double linear)
+{
+	if (linear <= 0.0) {
+		return Decibel::minimum;
+	}
+
+	return Decibel::from_linear(linear);
+}
+
+double AudioLevelMeter::power_to_lufs(double mean_square)
+{
+	if (mean_square <= 0.0) {
+		return Decibel::minimum;
+	}
+
+	// BS.1770 loudness uses K-weighted mean square. This first pass stores the
+	// compatible unit and can be extended with K-weighting without changing UI.
+	return -0.691 + 10.0 * std::log10(mean_square);
+}
+
+}