Fix machine envelope G-code emitting wrong limits due to broken extruder_id indexing (#12414)

print_machine_envelope() used get_extruder_id(extruder_id)*2 to index
machine limit arrays that only hold [Normal, Stealth] (2 entries).
For multi-extruder setups this went out-of-bounds, causing wrong M201/M203
values in the G-code which then override the estimator's correct limits.

Same class of bug as c6d1c11ebb but on the G-code writer side.

Changes:
- Remove unused extruder_id param from print_machine_envelope()
- Use .values.front() for M201/M203, matching M204/M205 in same function
- Change get_option_value() fallback from .back() to .front() so any
  future out-of-bounds index returns Normal mode instead of Stealth

src/libslic3r/GCode.cpp

@@ -2743,7 +2743,7 @@ void GCode::_do_export(Print& print, GCodeOutputStream &file, ThumbnailsGenerato
     m_pa_processor = std::make_unique<AdaptivePAProcessor>(*this, tool_ordering.all_extruders());
 
     // Emit machine envelope limits for the Marlin firmware.
-    this->print_machine_envelope(file, print, initial_extruder_id);
+    this->print_machine_envelope(file, print);
 
     // Disable fan.
     if (m_config.auxiliary_fan.value && print.config().close_fan_the_first_x_layers.get_at(initial_extruder_id)) {
@@ -3808,23 +3808,22 @@ PlaceholderParserIntegration &ppi = m_placeholder_parser_integration;
 
 // Print the machine envelope G-code for the Marlin firmware based on the "machine_max_xxx" parameters.
 // Do not process this piece of G-code by the time estimator, it already knows the values through another sources.
-void GCode::print_machine_envelope(GCodeOutputStream &file, Print &print, int extruder_id)
+void GCode::print_machine_envelope(GCodeOutputStream &file, Print &print)
 {
-    int matched_machine_limit_idx = get_extruder_id(extruder_id) * 2;
     const auto flavor = print.config().gcode_flavor.value;
     if ((flavor == gcfMarlinLegacy || flavor == gcfMarlinFirmware || flavor == gcfRepRapFirmware) &&
         print.config().emit_machine_limits_to_gcode.value == true) {
         int factor = flavor == gcfRepRapFirmware ? 60 : 1; // RRF M203 and M566 are in mm/min
         file.write_format("M201 X%d Y%d Z%d E%d\n",
-            int(print.config().machine_max_acceleration_x.values[matched_machine_limit_idx] + 0.5),
-            int(print.config().machine_max_acceleration_y.values[matched_machine_limit_idx] + 0.5),
-            int(print.config().machine_max_acceleration_z.values[matched_machine_limit_idx] + 0.5),
-            int(print.config().machine_max_acceleration_e.values[matched_machine_limit_idx] + 0.5));
+            int(print.config().machine_max_acceleration_x.values.front() + 0.5),
+            int(print.config().machine_max_acceleration_y.values.front() + 0.5),
+            int(print.config().machine_max_acceleration_z.values.front() + 0.5),
+            int(print.config().machine_max_acceleration_e.values.front() + 0.5));
         file.write_format("M203 X%d Y%d Z%d E%d\n",
-            int(print.config().machine_max_speed_x.values[matched_machine_limit_idx] * factor + 0.5),
-            int(print.config().machine_max_speed_y.values[matched_machine_limit_idx] * factor + 0.5),
-            int(print.config().machine_max_speed_z.values[matched_machine_limit_idx] * factor + 0.5),
-            int(print.config().machine_max_speed_e.values[matched_machine_limit_idx] * factor + 0.5));
+            int(print.config().machine_max_speed_x.values.front() * factor + 0.5),
+            int(print.config().machine_max_speed_y.values.front() * factor + 0.5),
+            int(print.config().machine_max_speed_z.values.front() * factor + 0.5),
+            int(print.config().machine_max_speed_e.values.front() * factor + 0.5));
 
         // Now M204 - acceleration. This one is quite hairy thanks to how Marlin guys care about
         // Legacy Marlin should export travel acceleration the same as printing acceleration.

src/libslic3r/GCode.hpp

@@ -640,7 +640,7 @@ private:
     double      calc_max_volumetric_speed(const double layer_height, const double line_width, const std::string co_str);
     std::string _extrude(const ExtrusionPath &path, std::string description = "", double speed = -1);
     bool _needSAFC(const ExtrusionPath &path);
-    void print_machine_envelope(GCodeOutputStream& file, Print& print, int extruder_id);
+    void print_machine_envelope(GCodeOutputStream& file, Print& print);
     void _print_first_layer_bed_temperature(GCodeOutputStream &file, Print &print, const std::string &gcode, unsigned int first_printing_extruder_id, bool wait);
     void _print_first_layer_extruder_temperatures(GCodeOutputStream &file, Print &print, const std::string &gcode, unsigned int first_printing_extruder_id, bool wait);
     // On the first printing layer. This flag triggers first layer speeds.

src/libslic3r/GCode/GCodeProcessor.cpp

@@ -124,7 +124,7 @@ static void set_option_value(ConfigOptionFloats& option, size_t id, float value)
 static float get_option_value(const ConfigOptionFloats& option, size_t id)
 {
     return option.values.empty() ? 0.0f :
-        ((id < option.values.size()) ? static_cast<float>(option.values[id]) : static_cast<float>(option.values.back()));
+        ((id < option.values.size()) ? static_cast<float>(option.values[id]) : static_cast<float>(option.values.front()));
 }
 
 static float estimated_acceleration_distance(float initial_rate, float target_rate, float acceleration)