Add "Snap to grid" for the selected symbols, pictures and texts

Edit > Aligner > Aligner sur la grille, also in the selection's
context menu and the command search, puts each selected item back where
dragging it would have left it: symbols and pictures on the folio grid,
free texts on the text grid. One undo step; wires follow their symbols.

Symbols leave the grid through the fine nudge (Alt+arrow, 1 px), and
nothing put them back: 508 of the 3,678 symbols in the example projects
are off the 10 px grid. First stage of discussion #1069.

The status bar says how many items moved, or that the selection was
already on the grid, and how many locked items were left in place.
Shapes are left out: they are made of several points and no single one
is the obvious one to snap.

The snap never reads the keyboard, unlike Diagram::snapToGrid(), so a
user who binds the action to a shortcut with Ctrl gets the grid, not
pixel rounding. The geometry is in alignment.h, tested by tst_alignment.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G2d2Zi8BfrYRPX88zhaoFG

sources/alignment.h

@@ -0,0 +1,49 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech 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 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech 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 QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#ifndef ALIGNMENT_H
+#define ALIGNMENT_H
+
+#include "textgrid.h"
+
+#include <QPointF>
+
+/**
+	The geometry behind the align commands, kept free of any scene so it
+	can be tested on its own.
+*/
+namespace Alignment
+{
+	/**
+		@return the movement that puts p on a grid of x_grid by y_grid,
+		divided by divisor as TextGrid::snap() does, or a null point when
+		p is already on it. Unlike Diagram::snapToGrid(), this never looks
+		at the keyboard: a command run from a shortcut with Ctrl in it must
+		not quietly round to the pixel instead.
+		Less than a millionth of a pixel counts as on the grid: positions
+		that went through arithmetic carry residues of that size, and
+		qFuzzyIsNull() (1e-12) is too strict to absorb them.
+	*/
+	inline QPointF gridOffset(const QPointF &p, int x_grid, int y_grid, qreal divisor = 1)
+	{
+		const QPointF offset = TextGrid::snap(p, x_grid, y_grid, divisor) - p;
+		auto clean = [](qreal v) { return qAbs(v) < 1e-6 ? 0.0 : v; };
+		return QPointF(clean(offset.x()), clean(offset.y()));
+	}
+}
+
+#endif // ALIGNMENT_H