CLI: --ground-* orientation from the Lay on Face planes, and --inspect-mesh (#15073)

* CLI: --ground-face-* / --lay-flat / --center-on-bed orientation primitives

Adds the CLI counterparts to the GUI's lay-flat / face-pick gizmos.
Scripted / CI / AI pipelines can now set orientation without rendering
a wxWidgets frame; today the only way is a GUI round-trip.

New CLI actions (all operate in the mesh-local frame so they compose
with prior --rotate-* / --orient flags):

  --ground-largest-face 1     Auto-detect the largest planar-face
   or  --lay-flat 1           cluster (area-weighted), rotate so its
                              normal points -Z. Covers "this part has
                              one obvious flat side" cases.

  --ground-face-normal NX,NY,NZ    Pick the face whose mesh-local
                                   normal best matches the given
                                   vector; ground it. e.g.
                                   `--ground-face-normal 1,0,0`
                                   stands a part on its +X side.

  --ground-face-point X,Y,Z        Find the triangle containing the
                                   given mesh-local point; ground its
                                   face. Disambiguates when several
                                   faces share a normal (largest
                                   containing triangle wins).

  --center-on-bed 1                Translate so the XY bounding-box
                                   centroid lands at the bed center
                                   (derived from printable_area).

New file `src/slic3r/Utils/MeshOrient.{hpp,cpp}`:
- collect_triangles_object / compute_face_clusters — quantize
  per-triangle normals (0.001, ~0.06°) and area-weighted-average
  within clusters. Same clustering logic used by lay-flat.
- apply_ground_rotation — same math as Selection::flattening_rotate
  in the GUI (Selection.cpp:1432): world-space quaternion from the
  transformed normal to -Z, applied as offset * new_rot * old_no_offset
  on every instance of every object, then a per-instance Z-lift so the
  grounded face lands at exactly 0 (avoids "No layers were detected"
  from FP-error z≈-1e-9).
- ground_face_point uses a top-N cluster search + point-in-triangle
  test in local space; largest-area triangle wins on ambiguity.

Rationale: without these, any CLI pipeline that needs a specific
face on the bed must either encode custom rotation math per part or
break out of the pipeline into the GUI. Both are bad for
reproducibility. The --ground-face-* triple + the largest-face
auto-mode cover essentially every orientation intent expressible
in a slicing wizard.

Scope:
- `src/slic3r/Utils/MeshOrient.{hpp,cpp}` — new, ~420 lines
- `src/slic3r/CMakeLists.txt` — 2-line registration
- `src/libslic3r/PrintConfig.cpp` — 5 new CLIMiscConfigDef entries
- `src/OrcaSlicer.cpp` — 58-line handler block + 1 include

No behaviour change when the flags are absent.

(cherry picked from commit c45a9795e1)

* CLI grounding: choose among the Lay on Face planes, per object

Addresses review:
- Move the geometry of GLGizmoFlatten::update_planes() into
  libslic3r/LayOnFace and use it from the gizmo and the CLI, so the
  --ground-* options pick convex-hull faces per object and instance,
  with part transformations (--rotate-x/y) applied.
- Drop --center-on-bed, the --lay-flat alias and MeshOrient; make
  --ground-largest-face a coBool.
- Parse --ground-face-normal and --ground-face-point strictly. A point
  that only some objects contain grounds those and leaves the others.
- Fold in --inspect-mesh from #14603, reporting the same planes.
- Tests in tests/libslic3r/test_lay_on_face.cpp: bounding boxes before
  and after, rotate then ground, two objects, and a ribbed part whose
  parallel inner faces outsum its base.

* CLI --inspect-mesh, --ground-face-*: reject missing input and empty values

- Without an input file or --load-assemble-list, --inspect-mesh printed
  nothing and exited 0. Reject it up front with CLI_INVALID_PARAMS.
- An explicit empty --ground-face-normal or --ground-face-point was
  silently ignored. Only options given on the command line reach the
  transforms loop, so an empty value now fails the strict parse like any
  other malformed value.

tests/libslic3r/CMakeLists.txt

@@ -37,6 +37,7 @@ add_executable(${_TEST_NAME}_tests
     test_triangle_selector.cpp
     test_meshboolean.cpp
     test_marchingsquares.cpp
+    test_lay_on_face.cpp
     test_model.cpp
     test_utils.cpp
     test_timeutils.cpp

tests/libslic3r/test_lay_on_face.cpp

@@ -0,0 +1,205 @@
+#include <catch2/catch_all.hpp>
+
+#include "libslic3r/LayOnFace.hpp"
+#include "libslic3r/Model.hpp"
+
+using namespace Slic3r;
+using Catch::Matchers::WithinAbs;
+
+namespace {
+
+// Adds a box part spanning `origin` to `origin + size`, in object coordinates.
+void add_box(ModelObject &object, const Vec3d &size, const Vec3d &origin = Vec3d::Zero())
+{
+    TriangleMesh mesh = make_cube(size.x(), size.y(), size.z());
+    mesh.translate(origin.cast<float>());
+    object.add_volume(std::move(mesh), ModelVolumeType::MODEL_PART, false);
+}
+
+ModelObject &add_box_object(Model &model, const Vec3d &size)
+{
+    ModelObject *object = model.add_object();
+    add_box(*object, size);
+    object->add_instance();
+    return *object;
+}
+
+// A 30 x 30 x 2 plate with three 1 mm thick, 20 mm tall ribs along Y. The rib sides facing -X add up
+// to more area than the plate's bottom, but only the bottom is a face of the convex hull.
+ModelObject &add_ribbed_plate(Model &model)
+{
+    ModelObject *object = model.add_object();
+    add_box(*object, { 30, 30, 2 });
+    for (double x : { 5., 14.5, 24. })
+        add_box(*object, { 1, 30, 20 }, { x, 0, 2 });
+    object->add_instance();
+    return *object;
+}
+
+std::vector<LayOnFacePlane> instance_planes(const ModelObject &object)
+{
+    return lay_on_face_planes(object, object.instances.front()->get_matrix_no_offset());
+}
+
+void lay_on_largest_face(ModelObject &object)
+{
+    const std::vector<LayOnFacePlane> planes = instance_planes(object);
+    const int                         idx    = find_largest_plane(planes);
+    REQUIRE(idx >= 0);
+    lay_on_face(object, 0, planes[idx].normal);
+}
+
+void check_size(const ModelObject &object, const Vec3d &expected)
+{
+    const Vec3d size = object.instance_bounding_box(0).size();
+    CHECK_THAT(size.x(), WithinAbs(expected.x(), 1e-3));
+    CHECK_THAT(size.y(), WithinAbs(expected.y(), 1e-3));
+    CHECK_THAT(size.z(), WithinAbs(expected.z(), 1e-3));
+}
+
+void check_on_bed(const ModelObject &object) { CHECK_THAT(object.instance_bounding_box(0).min.z(), WithinAbs(0., 1e-3)); }
+
+} // namespace
+
+TEST_CASE("A tilted box is laid on its largest face and dropped onto the bed", "[LayOnFace]")
+{
+    Model        model;
+    ModelObject &box = add_box_object(model, { 40, 20, 10 }); // the 40 x 20 faces are the largest
+    box.instances.front()->set_rotation({ 0.3, 0.5, 0.2 });
+    box.instances.front()->set_offset({ 0, 0, 50 });
+    REQUIRE(box.instance_bounding_box(0).size().z() > 11.);
+
+    const std::vector<LayOnFacePlane> planes = instance_planes(box);
+    REQUIRE(planes.size() == 6);
+    CHECK_THAT(planes.front().area, WithinAbs(40. * 20., 1e-2));
+
+    lay_on_largest_face(box);
+    CHECK_THAT(box.instance_bounding_box(0).size().z(), WithinAbs(10., 1e-3));
+    check_on_bed(box);
+}
+
+TEST_CASE("A box lying on one of its equally large faces is not flipped", "[LayOnFace]")
+{
+    // A half turn about X puts the other large face down, so the two cases expect different faces
+    // and neither can pass on the order in which the hull lists them.
+    const double rotation_x = GENERATE(0., PI);
+    Model        model;
+    ModelObject &box = add_box_object(model, { 40, 20, 10 }); // the bottom and top are both 40 x 20
+    box.instances.front()->set_rotation({ rotation_x, 0, 0 });
+    const Transform3d before = box.instances.front()->get_matrix_no_offset();
+
+    const std::vector<LayOnFacePlane> planes = instance_planes(box);
+    const int                         idx    = find_largest_plane(planes);
+    REQUIRE(idx >= 0);
+    // The face down on the plate is the object's -Z face, or its +Z face after the half turn.
+    CHECK_THAT(planes[idx].normal.z(), WithinAbs(rotation_x == 0. ? -1. : 1., 1e-6));
+
+    lay_on_face(box, 0, planes[idx].normal);
+    CHECK(box.instances.front()->get_matrix_no_offset().isApprox(before, 1e-9));
+}
+
+TEST_CASE("Faces are chosen from the orientation left by an earlier part rotation", "[LayOnFace]")
+{
+    Model        model;
+    ModelObject &box = add_box_object(model, { 40, 20, 10 });
+    box.rotate(PI / 2., X); // what --rotate-x 90 does: rotates the parts, not the instance
+    check_size(box, { 40, 10, 20 });
+
+    SECTION("the largest face") {
+        lay_on_largest_face(box);
+        check_size(box, { 40, 20, 10 });
+        check_on_bed(box);
+    }
+
+    SECTION("the face pointing along +X") {
+        const std::vector<LayOnFacePlane> planes = instance_planes(box);
+        const int                         idx    = find_plane_by_normal(planes, { 1, 0, 0 });
+        REQUIRE(idx >= 0);
+        CHECK_THAT(planes[idx].normal.x(), WithinAbs(1., 1e-6));
+        lay_on_face(box, 0, planes[idx].normal);
+        check_size(box, { 20, 10, 40 });
+        check_on_bed(box);
+    }
+}
+
+TEST_CASE("Objects are laid on their own faces independently", "[LayOnFace]")
+{
+    Model model;
+    // Standing on end through its instance rotation.
+    ModelObject &standing = add_box_object(model, { 40, 20, 10 });
+    standing.instances.front()->set_rotation({ 0, PI / 2., 0 });
+    // Standing on edge through a part rotation, lifted above the bed.
+    ModelObject &on_edge = add_box_object(model, { 30, 20, 5 });
+    on_edge.rotate(PI / 2., X);
+    on_edge.instances.front()->set_offset({ 100, 0, 30 });
+    check_size(standing, { 10, 20, 40 });
+    check_size(on_edge, { 30, 5, 20 });
+
+    for (ModelObject *object : model.objects)
+        lay_on_largest_face(*object);
+
+    check_size(standing, { 40, 20, 10 });
+    check_on_bed(standing);
+    check_size(on_edge, { 30, 20, 5 });
+    check_on_bed(on_edge);
+}
+
+TEST_CASE("A part rests on its largest hull face even when parallel inner faces add up to more area", "[LayOnFace]")
+{
+    Model        model;
+    ModelObject &plate = add_ribbed_plate(model);
+
+    double area_facing_minus_x = 0.;
+    for (const ModelVolume *volume : plate.volumes) {
+        const indexed_triangle_set &its = volume->mesh().its;
+        for (const Vec3i32 &face : its.indices) {
+            const Vec3d cross = (its.vertices[face[1]] - its.vertices[face[0]]).cast<double>().cross(
+                (its.vertices[face[2]] - its.vertices[face[0]]).cast<double>());
+            if (cross.normalized().x() < -0.999)
+                area_facing_minus_x += 0.5 * cross.norm();
+        }
+    }
+    // Summing triangle area per normal would pick a rib side over the 900 mm² bottom.
+    REQUIRE(area_facing_minus_x > 30. * 30.);
+
+    plate.instances.front()->set_rotation({ 0, PI / 2., 0 }); // stand the plate on its side
+    check_size(plate, { 22, 30, 30 });
+
+    const std::vector<LayOnFacePlane> planes = instance_planes(plate);
+    const int                         idx    = find_largest_plane(planes);
+    REQUIRE(idx >= 0);
+    CHECK_THAT(planes[idx].area, WithinAbs(30. * 30., 1e-2));
+    CHECK_THAT(planes[idx].normal.z(), WithinAbs(-1., 1e-6));
+
+    lay_on_face(plate, 0, planes[idx].normal);
+    check_size(plate, { 30, 30, 22 });
+    check_on_bed(plate);
+}
+
+TEST_CASE("Faces are selected in object coordinates whatever the instance rotation", "[LayOnFace]")
+{
+    Model        model;
+    ModelObject &plate = add_ribbed_plate(model);
+    plate.instances.front()->set_rotation({ 0, 0, PI / 2. });
+    const Transform3d                 instance_matrix = plate.instances.front()->get_matrix_no_offset();
+    const std::vector<LayOnFacePlane> planes          = lay_on_face_planes(plate, instance_matrix);
+    REQUIRE_FALSE(planes.empty());
+
+    // Every face center, as --inspect-mesh reports it, selects its own face.
+    for (size_t i = 0; i < planes.size(); ++i)
+        CHECK(find_plane_at_point(planes, instance_matrix, planes[i].center, 0.01) == int(i));
+
+    const int bottom = find_plane_at_point(planes, instance_matrix, { 15, 15, 0 }, 0.01);
+    REQUIRE(bottom >= 0);
+    CHECK_THAT(planes[bottom].normal.z(), WithinAbs(-1., 1e-6));
+    CHECK(find_plane_by_normal(planes, { 0, 0, -1 }) == bottom);
+    // Above the bottom plane, and on a rib side that lies inside the hull.
+    CHECK(find_plane_at_point(planes, instance_matrix, { 15, 15, 0.5 }, 0.01) == -1);
+    CHECK(find_plane_at_point(planes, instance_matrix, { 14.5, 15, 12 }, 0.01) == -1);
+}
+
+TEST_CASE("A part too small to rest on offers no faces", "[LayOnFace]")
+{
+    Model model;
+    CHECK(instance_planes(add_box_object(model, { 2, 2, 2 })).empty()); // every face is 4 mm², under the 5 mm² minimum
+}