R8 phase 1: extract app/engine shared utilities into shared/include/oakutil

Move the pure-header utilities shared by app/ and engine/ out of
engine/common/ into a new shared/include/oakutil/ layer (define, lerp,
decibel, digit, range, crashpadutils, autoscroll, qtutils, filefunctions
declarations, and a trimmed xmlutils exposing a CancelAtom-free void*
overload). engine/common/ keeps forwarding headers so internal include
paths are unchanged; app/ now includes oakutil/* directly.

engine/node/project.h gains an explicit NodeGroup forward declaration
previously obtained transitively through the old xmlutils.h.

shared/include/oakutil/lerp.h

@@ -0,0 +1,42 @@
+/***
+
+  Oak - Non-Linear Video Editor
+  Copyright (C) 2022 Olive Team
+  Modifications Copyright (C) 2025 mikesolar
+
+  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_LERP_H
+#define OAK_LERP_H
+
+template <typename T>
+/**
+ * @brief Linearly interpolate a value between a and b using t
+ *
+ * t should be a number between 0.0 and 1.0. 0.0 will return a, 1.0 will return b, and between will return a value
+ * in between a and b at that point linearly.
+ */
+T lerp(T a, T b, double t)
+{
+	return (a * (1.0 - t)) + (b * t);
+}
+
+template <typename T> T lerp(T a, T b, float t)
+{
+	return (a * (1.0f - t)) + (b * t);
+}
+
+#endif // OAK_LERP_H