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.

src/libslic3r/PrintConfig.cpp

@@ -11956,6 +11956,13 @@ CLIActionsConfigDef::CLIActionsConfigDef()
     def->tooltip = L("This outputs the model\u2019s information.");
     def->set_default_value(new ConfigOptionBool(false));
 
+    def = this->add("inspect_mesh", coBool);
+    def->label = L("Inspect mesh (JSON to stdout)");
+    def->tooltip = L("Print a JSON summary of each loaded object to stdout, then exit: its bounding boxes and the "
+                     "convex hull faces it can be laid on, with their normals, areas and centers. These are the faces "
+                     "the --ground-* options choose from. Machine-readable alternative to --info.");
+    def->set_default_value(new ConfigOptionBool(false));
+
     def = this->add("export_settings", coString);
     def->label = L("Export Settings");
     def->tooltip = L("This exports settings to a file. Use - to write them to stdout.");
@@ -12075,6 +12082,34 @@ CLITransformConfigDef::CLITransformConfigDef()
     def->sidetext = u8"°";	// degrees, don't need translation
     def->set_default_value(new ConfigOptionFloat(0));
 
+    // The --ground-* options choose from the faces the "Lay on Face" gizmo offers. Like the other
+    // transforms they run in command-line order, so they see the rotations given before them.
+    def = this->add("ground_largest_face", coBool);
+    def->label = L("Ground largest face");
+    def->tooltip = L("Lay each object on the largest face of its convex hull and drop it onto the bed. Of equally large "
+                     "faces, the one already facing down is kept. Objects without a face large enough to rest on are left "
+                     "as they are. Transforms run in command-line order, so rotations given before this option are respected. "
+                     "--orient 1 runs after all transforms and replaces the orientation.");
+    def->set_default_value(new ConfigOptionBool(false));
+
+    def = this->add("ground_face_normal", coString);
+    def->label = L("Ground face by normal");
+    def->tooltip = L("Lay each object on the convex hull face whose outward normal is closest to the direction NX,NY,NZ "
+                     "and drop it onto the bed. The direction is in object coordinates, which include the rotations given "
+                     "before this option and match the plate axes unless the input file rotates the object. For example, "
+                     "1,0,0 stands the object on its +X side. --orient 1 runs after all transforms and replaces the orientation.");
+    def->cli_params = "NX,NY,NZ";
+    def->set_default_value(new ConfigOptionString(""));
+
+    def = this->add("ground_face_point", coString);
+    def->label = L("Ground face at point");
+    def->tooltip = L("Lay each object on the convex hull face that contains the point X,Y,Z and drop it onto the bed. "
+                     "The point is in object coordinates, which include the rotations given before this option; "
+                     "--inspect-mesh reports face centers in them. Objects without such a face are left as they are, and "
+                     "the run fails if no object has one. --orient 1 runs after all transforms and replaces the orientation.");
+    def->cli_params = "X,Y,Z";
+    def->set_default_value(new ConfigOptionString(""));
+
     def = this->add("scale", coFloat);
     def->label = L("Scale");
     def->tooltip = L("Scale the model by a float factor.");