feat(gcode): resolve per-filament variant slots per layer

- the g-code writer tracks the current layer id and resolves
  FILAMENT_CONFIG/NOZZLE_CONFIG (plus every non-macro variant lookup,
  toolchange placeholder scalars, and the change-filament flush
  overrides) through Print's per-filament, per-layer config-index
  resolvers instead of the filament->extruder collapse
- update_layer_related_config refreshes the per-layer
  extruder/volume/nozzle maps in the writer config;
  update_placeholder_parser_with_variant_params remaps the
  filament-variant arrays into filament-id space for custom g-code
  (Orca's flush placeholder computation moves inside it)
- the engine's concrete per-filament volume assignment now merges into
  the config write-back (the temporary hold from the producer commit
  is lifted together with these consumers), and the background process
  reads the computed volume map back to the plate
- append_full_config dumps the resolved filament_map_2 slots
- update_used_filament_values gains a bounds guard
- tests: per-filament Hybrid slot resolution + null-result fallback

Result: on a Hybrid extruder, each filament's features slice with its
assigned sub-nozzle's variant values (speeds, volumetric limits,
retraction). Verified on a 4-filament H2C Hybrid project: outer walls
split into three feedrate populations (30/50/200 mm/s), toolpath
geometry byte-identical, deterministic across repeated slices. All 18
non-Hybrid reference fixtures stay byte-identical except the
filament_map_2 header value now showing the real slot. Auto grouping
ties (multiple zero-flush perfect matchings) may pick a different
filament-to-nozzle isolation than other slicers; verified co-optimal.

src/libslic3r/Print.cpp

@@ -2535,12 +2535,8 @@ void Print::process(long long *time_cost_with_cache, bool use_cache)
                     std::vector<int> base_volume_map = m_config.filament_volume_map.values;
                     if (base_volume_map.size() != derived_maps.size())
                         base_volume_map.assign(derived_maps.size(), (int)nvtStandard);
-                    // Orca: as on the non-sequential path, the engine's concrete volume
-                    // assignment (get_volume_map()) is deliberately not merged in until the
-                    // layer-aware g-code resolvers consume it; the config keeps its own
-                    // (GUI-injected or project) volume map, size-corrected above.
                     update_filament_maps_to_config(FilamentGroupUtils::update_used_filament_values(base_filament_map, derived_maps, used_filaments),
-                                                   base_volume_map,
+                                                   FilamentGroupUtils::update_used_filament_values(base_volume_map, grouping_result.get_volume_map(), used_filaments),
                                                    grouping_result.get_nozzle_map());
                 }
             }