color: global LUT library, LUT error reporting, clamp + display fixes

- Add a global LUT library: user-configurable directories (new
  Preferences > LUT tab) scanned recursively for .cube/.3dl files; LUT
  node file pickers offer the library dirs as sidebar shortcuts via a
  'lut_library' input property handled by the param view bridge
- OCIOLutNode no longer fails silently: missing files, unsupported
  extensions and OCIO load errors are recorded in last_error() and
  surfaced in the status bar (input still passes through for rendering
  safety)
- ColorDialog: re-enable the display -> reference conversion using
  ColorProcessor::kInverse with a validity guard, and re-enable the
  Display tab in ColorValuesWidget; covered by a round-trip regression
  test proving the old OCIO inverse crash no longer occurs
- OCIOGradingTransformLinearNode: enforce the OCIO clampWhite >
  clampBlack invariant per frame in Value() so keyframed/connected
  values cannot produce invalid grading transforms, and constrain the
  white clamp UI minimum whenever the black clamp is static
- Regression tests for LUT extension checks, direction switching, node
  error reporting, LUT library scanning, display inverse round-trip and
  clamp enforcement

app/render/lutlibrary.cpp

@@ -0,0 +1,86 @@
+/***
+
+  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 "lutlibrary.h"
+
+#include <QDir>
+#include <QDirIterator>
+#include <QFileInfo>
+
+#include "config/config.h"
+
+namespace olive
+{
+
+bool LUTLibrary::IsSupportedExtension(const QString &suffix)
+{
+	QString s = suffix;
+	if (s.startsWith(QLatin1Char('.'))) {
+		s.remove(0, 1);
+	}
+
+	const QString lower = s.toLower();
+	return lower == QStringLiteral("cube") || lower == QStringLiteral("3dl");
+}
+
+QStringList LUTLibrary::GetDirectories()
+{
+	const QString serialized = OLIVE_CONFIG("LUTLibraryPaths").toString();
+
+	QStringList dirs = serialized.split(QLatin1Char(';'), Qt::SkipEmptyParts);
+	for (QString &dir : dirs) {
+		dir = QDir::fromNativeSeparators(dir.trimmed());
+	}
+	return dirs;
+}
+
+void LUTLibrary::SetDirectories(const QStringList &dirs)
+{
+	QStringList cleaned;
+	for (const QString &dir : dirs) {
+		const QString trimmed = dir.trimmed();
+		if (!trimmed.isEmpty() && !cleaned.contains(trimmed)) {
+			cleaned.append(trimmed);
+		}
+	}
+
+	Config::Current()[QStringLiteral("LUTLibraryPaths")] =
+		cleaned.join(QLatin1Char(';'));
+}
+
+QStringList LUTLibrary::GetLutFiles()
+{
+	QStringList files;
+
+	static const QStringList kFilters = { QStringLiteral("*.cube"),
+										  QStringLiteral("*.3dl") };
+
+	for (const QString &dir : GetDirectories()) {
+		QDirIterator it(dir, kFilters, QDir::Files,
+						QDirIterator::Subdirectories);
+		while (it.hasNext()) {
+			files.append(it.next());
+		}
+	}
+
+	return files;
+}
+
+}