Merge upstream main: color mixing feature and related fixes

Brings in 54 upstream commits, the bulk of them the BambuStudio-ported color
mixing / mixed filament subsystem (#15347) plus its follow-ups, along with the
Assimp-backed colored OBJ import, warning-policy build changes, and assorted
profile and localization updates.

Two conflicts, both "each side added at the same point", resolved by keeping
both:

- Print::validate() -- our IMEX multi-color block and upstream's new gradient
  mixed filament warning were inserted at the same spot after the empty
  extruders check. They test unrelated conditions, so both are kept, each with
  its own closing brace.
- tests/libslic3r/test_3mf.cpp -- our three IMEX per-plate round-trip scenarios
  and upstream's mixed-filament round-trip scenario both append to the end of
  the file, and each side added one include. All four scenarios and both
  includes are kept.

Everything else merged cleanly, including GCode.cpp, ToolOrdering.cpp,
PartPlate.cpp and PrintConfig.cpp. Upstream left the is_extruder_used block
untouched, so the IMEX supplement still applies, and estimate_wipe_tower_polygon
is unchanged, so the prime tower hull work is unaffected.

Not addressed here, and worth its own change: a mixed filament is a virtual slot
that no nozzle carries, while physical_extruder_map routes logical slots to
physical heads. Print::extruders() lists mixed slots under their own id whereas
tool_ordering.all_extruders() lists them post-expansion, so the IMEX pem lookups
have no defined answer for a mixed slot. Upstream's own guards reject a mixed
filament where a physical slot is required; IMEX likely wants the same.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/slic3r/GUI/GCodeViewer.cpp

@@ -423,7 +423,7 @@ void GCodeViewer::SequentialView::Marker::render_position_window(const libvgcode
         if (properties_shown) {
             float label_w = 0.0f;
             float value_w = 0.0f;
-            properties_rows.reserve(13);
+            properties_rows.reserve(14);
             auto add_row = [&properties_rows, &label_w, &value_w](std::string label, std::string value) {
                  label_w = std::max(label_w, ImGui::CalcTextSize(label.c_str()).x);
                  value_w = std::max(value_w, ImGui::CalcTextSize(value.c_str()).x);
@@ -436,6 +436,27 @@ void GCodeViewer::SequentialView::Marker::render_position_window(const libvgcode
             add_row(_u8L("Width"), buff);
             if (is_extrusion) sprintf(buff, ("%.3f " + _u8L("mm")).c_str(), vertex.height); else strcpy(buff, NA_CSTR);
             add_row(_u8L("Height"), buff);
+            // ORCA: Length of the move ending at the current vertex. Arc moves (G2/G3) are discretized
+            // into several vertices sharing the same gcode line id, so accumulate the whole run to report
+            // the arc length instead of the length of a single chord.
+            if (vertex_id > 0 && (is_extrusion || vertex.is_travel() || vertex.is_wipe())) {
+                const size_t vertices_count = viewer->get_vertices_count();
+                size_t first_id = vertex_id;
+                while (first_id > 0 && viewer->get_vertex_at(first_id - 1).gcode_id == vertex.gcode_id)
+                    --first_id;
+                size_t last_id = vertex_id;
+                while (last_id + 1 < vertices_count && viewer->get_vertex_at(last_id + 1).gcode_id == vertex.gcode_id)
+                    ++last_id;
+                float length = 0.0f;
+                for (size_t i = std::max<size_t>(first_id, 1); i <= last_id; ++i) {
+                    length += (libvgcode::convert(viewer->get_vertex_at(i).position) -
+                               libvgcode::convert(viewer->get_vertex_at(i - 1).position)).norm();
+                }
+                sprintf(buff, ("%.3f " + _u8L("mm")).c_str(), length);
+            }
+            else
+                strcpy(buff, NA_CSTR);
+            add_row(_u8L("Length"), buff);
             sprintf(buff, "%d", vertex.layer_id + 1);
             add_row(_u8L("Layer"), buff);
             sprintf(buff, ("%.1f " + _u8L("mm/s")).c_str(), vertex.feedrate);