feat: Add iXex parallel printing support for IDEX and IQEX printers

Introduces first-class parallel printing (copy/mirror modes) for printers
with multiple independent X-axis carriages. Branded iXex (independent X
extruder), targeting Klipper firmware with a firmware-agnostic design.

- PrintConfig: new printer options declaring iXex capability and geometry
  (is_ixex, ixex_gantry_count, ixex_tools_per_gantry, carriage dims,
  tool layout, and per-mode name/role/gcode arrays)
- Preset: iXex keys registered in printer and process preset option lists
- Tab: IXexModesCtrl visual grid editor in Printer preset tab; mode
  dropdown in Process → Others tab; clear_pages() nulls iXex pointers to
  prevent dangling-pointer crash on preset save
- PartPlate: 2D zone visualization (active/dimmed/dividers) and
  placement-violation detection (has_ixex_placement_violations) that
  blocks slicing when objects fall outside the primary zone
- Plater: violation detection wired into update_background_process so
  the Slice button is disabled with an error notification on violation
- GCode: mode-activation G-code injected before machine_start_gcode
- GCodeViewer: multi-carriage toolhead markers in sequential preview,
  filament legend annotated with active carriage count and mode name

src/libslic3r/GCode.cpp

@@ -3054,6 +3054,24 @@ void GCode::_do_export(Print& print, GCodeOutputStream &file, ThumbnailsGenerato
             file.write(m_writer.set_chamber_temperature(max_chamber_temp, true)); // set chamber_temperature
     }
 
+    // iXex: inject mode G-code before machine_start_gcode so the carriage mode
+    // macro is active before any motion, homing, or probing in start G-code.
+    if (print.config().is_ixex.value && !print.objects().empty()) {
+        const std::string& ixex_mode = print.objects().front()->config().ixex_parallel_mode.value;
+        if (!ixex_mode.empty() && ixex_mode != "primary") {
+            const auto& mode_names  = print.config().ixex_mode_names.values;
+            const auto& mode_gcodes = print.config().ixex_mode_gcodes.values;
+            for (size_t i = 0; i < mode_names.size() && i < mode_gcodes.size(); ++i) {
+                if (mode_names[i] == ixex_mode && !mode_gcodes[i].empty()) {
+                    std::string ixex_gcode = this->placeholder_parser_process(
+                        "ixex_mode_gcode", mode_gcodes[i], initial_extruder_id);
+                    file.writeln(ixex_gcode);
+                    break;
+                }
+            }
+        }
+    }
+
     // Write the custom start G-code
     file.writeln(machine_start_gcode);