fix: out-of-bounds read computing tool-ordering max layer height (#14665)

* fix: out-of-bounds read computing tool-ordering max layer height

calc_max_layer_height() loops over the extruder count (nozzle_diameter)
but indexes max_layer_height with the same counter, reading past the end
when that array is shorter. Silent on release builds, aborts under a
bounds-checked STL (_GLIBCXX_ASSERTIONS).

Read via get_at(), which falls back to the first entry when the index is
out of range, as Slicing.cpp already does for this option.

Add a fff_print regression test slicing a two-extruder printer with a
single-entry max_layer_height.

* docs: clarify how max_layer_height ends up short in the regression test

Normalization sizes it to the filament count under single_extruder_multi_material,
not "a mismatch a profile can ship" as the earlier comment guessed.

src/libslic3r/GCode/ToolOrdering.cpp

@@ -138,7 +138,8 @@ static double calc_max_layer_height(const PrintConfig &config, double max_object
 {
     double max_layer_height = std::numeric_limits<double>::max();
     for (size_t i = 0; i < config.nozzle_diameter.values.size(); ++ i) {
-        double mlh = config.max_layer_height.values[i];
+        // max_layer_height may be shorter than the extruder count; get_at() clamps.
+        double mlh = config.max_layer_height.get_at(i);
         if (mlh == 0.)
             mlh = 0.75 * config.nozzle_diameter.values[i];
         max_layer_height = std::min(max_layer_height, mlh);