A hole square to a folded sheet metal panel is flat machining (#656)

A hole drilled square to a flange's panel, not only the base's, is now a flat cut-out of the sheet: it unfolds with its panel to where the bend allowance puts it, shows in the flat pattern and DXF as a circle, and survives unfold and refold. A pocket tool spans the cut-outs already inside it (a counterbore's hole), so the order they are made in no longer matters. Fixes #560.

docs/book/src/guide/sheet-metal.md

@@ -179,6 +179,12 @@ A Box cut, or a Cylinder cut whose axis is the sheet's normal, machines the shee
 extrude of its footprint: through the sheet it is a cut-out (a cylinder a round one), and
 part way it is a pocket. A primitive cut at any other angle, or across a folded bend, is refused.
 
+Hole machines the sheet the same way from a flat face of any panel, the base or a flange, on
+either side, folded or unfolded: square to that panel, a through hole (plain or tapped) is a
+round cut-out of the flat pattern and a `CIRCLE` in the DXF, and a counterbore or a blind
+flat-bottomed hole is a round pocket. A hole slanted to its panel or drilled into the edge of
+the sheet, a countersink and a drill point are refused.
+
 Parallel, untapered extrude cuts also preserve shallow pockets through Unfold / Refold.
 The pocket floor and side walls remain solid geometry; overlapping cuts from opposite sides
 become a through hole wherever they remove the whole thickness. Flat-pattern JSON lists