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/PrintConfig.hpp

@@ -774,7 +774,7 @@ extern std::set<std::string> printer_options_with_variant_2;
 extern std::set<std::string> empty_options;
 
 extern void compute_filament_override_value(const std::string& opt_key, const ConfigOption *opt_old_machine, const ConfigOption *opt_new_machine, const ConfigOption *opt_new_filament, const DynamicPrintConfig& new_full_config,
-    t_config_option_keys& diff_keys, DynamicPrintConfig& filament_overrides, std::vector<int>& f_maps);
+    t_config_option_keys& diff_keys, DynamicPrintConfig& filament_overrides, std::vector<int>& f_map_indices);
 
 void handle_legacy_sla(DynamicPrintConfig &config);
 
@@ -1447,8 +1447,12 @@ PRINT_CONFIG_CLASS_DEFINE(
     ((ConfigOptionInts,                required_nozzle_HRC))
     ((ConfigOptionEnum<FilamentMapMode>, filament_map_mode))
     ((ConfigOptionInts,                filament_map))
+    ((ConfigOptionInts,                filament_volume_map))
+    ((ConfigOptionInts,                filament_nozzle_map))
+    ((ConfigOptionInts,                filament_map_2)) //used for multi nozzle, map filament to the index identified by extruder+nozzle_volume_type
     //((ConfigOptionInts,                filament_extruder_id))
     ((ConfigOptionStrings,             filament_extruder_variant))
+    ((ConfigOptionInts,                filament_self_index))
     ((ConfigOptionBool,                support_object_skip_flush))
     ((ConfigOptionEnum<BedTempFormula>, bed_temperature_formula))
     ((ConfigOptionInts,                physical_extruder_map))