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/GCode/GCodeProcessor.cpp

@@ -6940,10 +6940,12 @@ float GCodeProcessor::minimum_travel_feedrate(PrintEstimatedStatistics::ETimeMod
 }
 
 // Machine limit arrays hold 2 values: [0]=Normal, [1]=Stealth. Index by mode only.
-// BambuStudio used extruder_id*2+mode to support per-nozzle limits, but OrcaSlicer
-// never ported that system (filament_map_2 / get_config_idx_for_filament), so the
-// extruder_id offset was always wrong: uninitialized extruder (255) or extruder > 0
-// would overshoot the array and fall back to values.back() (stealth limits).
+// Orca: per-(extruder x volume-type) machine limits are deliberately not resolved here even
+// though the slicing side now materializes filament_map_2 and the per-filament volume map
+// (an extruder_id*2+mode style offset would need filament_map_2 in the processor-side config
+// plus a re-audit of every get_option_value(..., mode) call). This only affects
+// time-estimation fidelity: limits are mode-indexed for ALL multi-extruder printers alike, so
+// a Hybrid extruder degrades no further than existing dual-extruder machines. Follow-up.
 float GCodeProcessor::get_axis_max_feedrate(PrintEstimatedStatistics::ETimeMode mode, Axis axis) const
 {
     switch (axis)