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/PrintObject.cpp

@@ -1171,12 +1171,37 @@ bool PrintObject::has_belt_brim() const
         return false;
     if (m_config.brim_type == btNoBrim)
         return false;
-    if (m_config.brim_width.value <= 0. && m_config.leading_brim_length.value <= 0.
-        && m_config.extra_brim_width.value <= 0.)
+    // An inner-only brim has no leading/extra geometry: leading_brim_length and
+    // extra_brim_width both widen the OUTER ring, which btInnerOnly never emits, so it
+    // produces nothing unless brim_width itself is positive.  Every other brim type is
+    // satisfied by any one of the three widths.  Requiring the width here (instead of
+    // "any width") stops has_belt_brim() - and therefore Print::validate() - from
+    // rejecting the prime tower / spiral vase for a brim that would never be drawn.
+    if (m_config.brim_type == btInnerOnly) {
+        if (m_config.brim_width.value <= 0.)
+            return false;
+    } else if (m_config.brim_width.value <= 0. && m_config.leading_brim_length.value <= 0.
+               && m_config.extra_brim_width.value <= 0.) {
         return false;
+    }
     return ! this->has_raft();
 }
 
+unsigned int PrintObject::belt_brim_filament() const
+{
+    // 1-based, matching PrintRegion::outer_wall_filament_id and the raw values pushed
+    // into LayerTools::extruders in ToolOrdering::collect_extruders (the whole list is
+    // reindexed to 0-based later).  Lowest positive outer-wall filament over the
+    // printing regions; 1 when none is explicitly set.
+    unsigned int brim_filament = 0;
+    for (size_t i = 0; i < this->num_printing_regions(); ++ i) {
+        const unsigned int f = this->printing_region(i).config().outer_wall_filament_id.value;
+        if (f > 0 && (brim_filament == 0 || f < brim_filament))
+            brim_filament = f;
+    }
+    return brim_filament == 0 ? 1u : brim_filament;
+}
+
 bool PrintObject::belt_brim_instances_compatible() const
 {
     // One set of bands is shared by every instance of this object, so they must all sit at