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.

engine/common/digit.h

@@ -1,47 +1,2 @@
-/***
-
-  Olive - 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_DIGIT_H
-#define OAK_DIGIT_H
-
-#include <stdint.h>
-
-namespace olive
-{
-
-inline int64_t get_digit_count(int64_t input)
-{
-	input = std::abs(input);
-
-	int64_t lim = 10;
-	int64_t digit = 1;
-
-	while (input >= lim) {
-		lim *= 10;
-		digit++;
-	}
-
-	return digit;
-}
-
-}
-
-#endif // OAK_DIGIT_H
+// Forwarding header - canonical content moved to shared/include/oakutil/digit.h
+#include "oakutil/digit.h"