Organic supports: restore the per-slice collision, bed and belt trim

dda58b07cd stripped debug instrumentation from TreeSupport3D.cpp with a
script, and that script also deleted the loop in organic_draw_branches()
that trims every branch slice against the collision volume, the bed and,
on a belt, the belt plane.  This is the generator every printer uses, not
a belt code path, and it is the one place where raistlin7447's export
fixtures differed from main with belt printing off.  Restore the loop as
it was on main, with the belt-floor clip.

The new test prints a cube carrying a 60 mm plate with organic supports
on a flat-bed printer and checks on every support layer that no support
extrusion comes within 0.2 mm of the part's slice.  It guards that
invariant; on this fixture the loop's own effect is a sub-millimetre
reshaping of one branch (verified by slicing the fixture with and
without the loop), below the asserted gap, so the test does not by
itself fail without the loop.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/libslic3r/Support/TreeSupport3D.cpp

@@ -3952,6 +3952,13 @@ void organic_draw_branches(
                     const double tiny_area = tiny_area_threshold();
                     //FIXME parallelize?
                     for (LayerIndex i = 0; i < LayerIndex(slices.size()); ++i) {
+                        // ORCA: safety offset when trimming collision/bed to improve robustness.
+                        slices[i] = diff_clipped(slices[i], volumes.getCollision(0, layer_begin + i, true), ApplySafetyOffset::Yes); // FIXME parent_uses_min || draw_area.element->state.use_min_xy_dist);
+                        slices[i] = intersection(slices[i], volumes.m_bed_area, ApplySafetyOffset::Yes);
+                        // Belt floor: clip branch slices against the belt surface plane.
+                        LayerIndex belt_idx = layer_begin + i;
+                        if (belt_idx < LayerIndex(volumes.m_belt_floor.size()) && !volumes.m_belt_floor[belt_idx].empty())
+                            slices[i] = diff(slices[i], volumes.m_belt_floor[belt_idx]);
                         remove_small(slices[i], tiny_area);
                     }