Align: a group reduced to one shape lines up on its own centre

A group whose only unlocked member is a shape (its other members
locked) kept a centre of (0,0), so Centrer horizontalement and
Centrer verticalement pulled every item toward the folio origin and
the shape itself landed off the line. The rule that turns a group's
members into one item now lives in Alignment::combined(), where every
member, shapes included, brings its own centre, and tst_alignment
covers it.

Also comments why unitFor()'s returned reference is safe, as asked in
the review of #1087.

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

sources/alignment.h

@@ -54,6 +54,24 @@ namespace Alignment
 		QPointF ref;
 	};
 
+	/**
+		@return @a items taken as one piece, the way a group lines up:
+		their edges together, and the middle of that box as its centre.
+		A single item keeps its own centre.
+	*/
+	inline Item combined(const QList<Item> &items)
+	{
+		Item result;
+		for (const Item &item : items)
+			result.edges = result.edges.isNull() ? item.edges
+							     : result.edges.united(item.edges);
+		if (items.size() == 1)
+			result.ref = items.first().ref;
+		else
+			result.ref = result.edges.center();
+		return result;
+	}
+
 	/**
 		@return true if aligning on @a edge moves items along x
 	*/