feat(config): keep per-(extruder x volume-type) slots in variant expansion

- get_extruder_nozzle_volume_count derives per-extruder volume-type slot
  lists from extruder_nozzle_stats (absent stats = one slot per extruder)
- update_values_to_printer_extruders learns the slot layout: when any
  extruder mixes volume types, option arrays keep one slot per
  (extruder x volume type), extruder-ascending then volume-ascending;
  single-slot resolution takes the filament's volume type on mixed
  extruders
- update_values_to_printer_extruders_for_multiple_filaments applies a
  per-filament nozzle_volume_type override from filament_volume_map
  (when sized to the filament count) and remaps filament_self_index
  through the same pipeline as every other filament key
- get_config_index_base + is_auto_filament_map_mode helpers (consumers
  land with the per-filament config-index resolvers)
- callers updated: PresetBundle composition paths, PrintApply (counts
  hoisted above the extruder_applied guard), Print write-back
- new tests: slot counting, Hybrid slot expansion incl. stride 2,
  per-filament override, non-Hybrid degeneracy

Non-Hybrid printers keep their variant layout and values (proven by a
19-fixture byte gate; the only header delta is filament_self_index now
flowing through the same variant pipeline as its sibling filament
keys). Hybrid slices grow the config-block variant arrays to one entry
per sub-nozzle volume type; motion g-code is unchanged until the
g-code writer consumes the new slots.

tests/libslic3r/CMakeLists.txt

@@ -12,6 +12,7 @@ add_executable(${_TEST_NAME}_tests
     test_clipper_offset.cpp
     test_clipper_utils.cpp
     test_config.cpp
+    test_config_variant_expansion.cpp
     test_toolordering_nozzle_group.cpp
     test_preset_bundle_loading.cpp
     test_preset_setting_id.cpp