Fix belt brim emission: dropped first-contact bands, tool selection, inner-only predicate (A,B,C,D)

- Emit coincident belt_brim_by_layer bands even when the leading object layer
  has no InstanceVisit (zero-extrusion lead-in / no coinciding support), so the
  brim at first belt contact is no longer dropped.
- Register each coincident band's brim filament in ToolOrdering and emit each
  band exactly once, in its brim-filament pass; emit ordinary-layer aprons in
  the brim pass before object extrusion (correct tool, brim-first) instead of
  with whatever tool was active.
- has_belt_brim(): inner-only brims need brim_width>0 (leading/extra produce no
  inner geometry), fixing spurious prime-tower/spiral rejection; mirror in
  wants_brim. Single-extruder/single-object output is unchanged except
  previously-dropped bands now print.

src/libslic3r/Print.cpp

@@ -1374,9 +1374,15 @@ StringObjectException Print::validate(std::vector<StringObjectException> *warnin
 
         for (const PrintObject *object : m_objects) {
             const PrintObjectConfig &ocfg = object->config();
+            // Mirror PrintObject::has_belt_brim(): an inner-only brim needs a positive
+            // brim_width (leading/extra widen only the outer ring), so keep this
+            // predicate in step or the belt-brim warnings below would fire for a brim
+            // that has_belt_brim() rejects.
             const bool wants_brim = ocfg.brim_type != btNoBrim
-                                 && (ocfg.brim_width.value > 0. || ocfg.leading_brim_length.value > 0.
-                                     || ocfg.extra_brim_width.value > 0.);
+                                 && (ocfg.brim_type == btInnerOnly
+                                         ? ocfg.brim_width.value > 0.
+                                         : (ocfg.brim_width.value > 0. || ocfg.leading_brim_length.value > 0.
+                                            || ocfg.extra_brim_width.value > 0.));
             if (! wants_brim)
                 continue;