belt: correct the designed-view preview's belt-Z origin and reject mis-mapped outliers

The Cartesian designed-view preview over-extended the toolpaths past the model
shell by a height-proportional amount (up to ~20mm tall parts), most visibly on
long multi-part prints; compact parts like a calibration cube looked fine.

Two coupled causes:
- Belt start G-code that primes with a Z advance and a 'G92 Z0' reset leaves a
  constant machine-Z origin in the GCodeProcessor, so move positions are stored as
  gcode_Z + origin. The linear back-transform mixes that constant with the
  gantry-Y term, leaving a per-move designed-Y error that min-corner anchoring
  cannot cancel when an elevated move (e.g. a bridge) happens to cancel it at the
  bbox minimum. Expose GCodeProcessorResult::belt_z_origin (the m_origin[Z] left by
  the start G-code) and subtract it before the back-transform.
- Elevated features (bridges/overhangs) are mis-mapped by the linear inverse to
  outside the model body; build the anchor bbox only from moves within model_bb +/-
  10mm, with a fallback to the full bbox when the clip would drop the bulk (object
  placed away from the belt entry) so the gross-offset case still anchors.

Preview-only; G-code output is unchanged.

src/libslic3r/GCode/GCodeProcessor.cpp

@@ -4933,6 +4933,13 @@ void GCodeProcessor::process_G92(const GCodeReader::GCodeLine& line)
     if (line.has_z()) {
         m_origin[Z] = m_end_position[Z] - line.z() * lengths_scale_factor;
         any_found = true;
+        // Belt only: the start G-code's purge-blob advance + G92 Z0 resets leave a constant
+        // machine-Z origin offset here; the designed-view back-transform subtracts it so
+        // toolpaths map to the model's belt coordinate (gcode Z). Gated on belt_tilt_angle
+        // (set from the belt header, parsed before the body) so non-belt G-code processing
+        // is byte-identical — no unconditional work on the shared path.
+        if (m_result.belt_tilt_angle != 0.f)
+            m_result.belt_z_origin = m_origin[Z];
     }
 
     if (line.has_e()) {