Design: fix hole placement (top-face default) + invisible internal thread

Hole: with no face explicitly picked, the tool fell back to the XY datum at
z=0 (the model's underside), so placing a hole from a top view read parallax-
shifted. Default to the solid's top face (top_face_index_of) so the footprint
sits on the surface being viewed; the XY/XZ/YZ dropdown still overrides.

Internal thread: the bore was re-cut at the nominal radius, which coincides
with an existing hole's wall — the coincident faces fouled the groove boolean
so it removed ~nothing (invisible thread). Cut the bore at the minor diameter
(radius - depth) instead: strictly inside any existing wall, leaving it clean
for the groove; on solid stock it forms the tap-drill.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BVzKmX6Y1aEteit1HTXG4Q

src/libslic3r/CadDocument.cpp

@@ -1406,7 +1406,13 @@ void CadDocument::apply_feature(TopoDS_Shape& result, bool& have_body,
             // OUTWARD helical groove into its wall. When the thread is invoked on
             // an existing hole the bore cut is coincident (a no-op that may report
             // !IsDone) — tolerate it so the visible groove cut below still runs.
-            TopoDS_Shape bore = BRepPrimAPI_MakeCylinder(ax2, f.thread_radius,
+            // Cut the pocket at the MINOR diameter (radius - depth), not the nominal radius.
+            // A nominal-radius bore that coincides with an existing hole's wall creates
+            // coincident faces that foul the following groove boolean (the groove then removes
+            // ~nothing -> invisible thread). The minor bore stays strictly inside any existing
+            // hole wall, leaving it clean for the groove; on solid stock it forms the tap-drill.
+            const double bore_r = std::max(0.5, f.thread_radius - f.thread_depth);
+            TopoDS_Shape bore = BRepPrimAPI_MakeCylinder(ax2, bore_r,
                                                          f.thread_height).Shape();
             try {
                 BRepAlgoAPI_Cut cut_bore(result, bore);