feat(print): add per-filament config-index resolvers and filament_map_2

- Print::get_nozzle_config_index / get_filament_config_indx resolve a
  filament's variant slot per layer from the nozzle group result, with
  hashed index caches; when no group result is published (sequential
  prints), they fall back to the static filament->extruder mapping so
  behavior is unchanged
- filament_map_2 caches each filament's resolved print-variant slot;
  rebuilt in Print::apply after the filament_map diff handling and in
  the filament-map write-back
- filament retract overrides now key by slot indices: apply_override
  fallback indexing flips to 0-based, Print::apply passes
  filament_map/extruder indices, the write-back passes filament_map_2
  (identical resolution while slots equal extruders)
- filament_volume_map/filament_nozzle_map/filament_map_2/
  filament_self_index become PrintConfig static members (required for
  member access); grouping input guards tightened so their registered
  1-element defaults are never mistaken for real per-filament maps
  (single-filament manual mode keeps the mix-marker fallback)
- update_filament_self_index_cache refreshed at every full-config
  assignment
- tests: 0-based apply_override fallback, get_config_index_base
  hit/miss/mixed-type cases

The resolvers are not consumed by the g-code writer yet. Non-Hybrid
g-code is unchanged except the config header, which now serializes the
three new static keys (defaults until the per-filament producer lands);
verified by the 19-fixture byte gate: 3 added header lines per fixture,
zero motion changes.

src/libslic3r/Config.hpp

@@ -716,6 +716,7 @@ public:
     	return false;
     }
     // Apply an override option, possibly a nullable one.
+    //default_index are 0 based
     bool apply_override(const ConfigOption *rhs, std::vector<int>& default_index) override {
         if (this->nullable())
         	throw ConfigurationError("Cannot override a nullable ConfigOption.");
@@ -751,8 +752,8 @@ public:
                 this->values[i] = rhs_vec->values[i];
                 modified        = true;
             } else {
-                if ((i < default_index.size()) && (default_index[i] - 1 < default_value.size()))
-                    this->values[i] = default_value[default_index[i] - 1];
+                if ((i < default_index.size()) && (default_index[i] < default_value.size()))
+                    this->values[i] = default_value[default_index[i]];
                 else
                     this->values[i] = default_value[0];
             }