belt: replace height-check skip with a belt-correct vertical-clearance check

The original PR skipped the max-print-height check entirely on belt printers
because the sliced (virtual) Z is belt travel, not build height. As the reviewer
noted, that removed the only working height guard. Restore a correct guard:

- Print::validate: on belt printers, compare the upright object height
  (max over instances of the scene-space bbox) against printable_height directly.
  printable_height is the usable VERTICAL clearance above the belt: the gantry
  travels up the tilted plane (reach = height/cos(tilt)) and its axis range is
  sized for that (IR3 V2: ~354 mm gantry travel = 250 mm vertical at 45deg, and
  printable_height = 250). Hardware-confirmed 250 mm vertical clearance, so no
  cos(tilt) factor is applied.
- BuildVolume::set_belt_printer: drop the diagonal Z scaling; the build-volume Z
  already equals printable_height, keeping the live 'outside build volume'
  highlight in agreement with validate().

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/libslic3r/BuildVolume.cpp

@@ -193,12 +193,11 @@ void BuildVolume::set_belt_printer(bool enabled, double angle_deg, bool infinite
             // Extend the Y bound to a very large value for infinite belt.
             m_bboxf.max.y() = 100000.;
         }
-        // Adjust max print height to diagonal reach: printable_height / sin(belt_angle).
-        if (angle_deg > 0. && angle_deg < 90.) {
-            double sin_a = std::sin(angle_deg * M_PI / 180.0);
-            if (sin_a > 0.)
-                m_bboxf.max.z() = m_max_print_height / sin_a;
-        }
+        // Belt printer: the Z extent already equals printable_height (set above), which
+        // is the usable vertical clearance above the belt. The gantry's axis range is
+        // sized to reach height/cos(tilt), so no diagonal scaling is applied here — this
+        // keeps the live "outside build volume" highlight in agreement with Print::validate().
+        (void) angle_deg;
     }
 }