feat(engine): write per-variant filament values back for selector regroups

When the per-layer filament selector (enable_filament_dynamic_map)
migrates a filament across nozzle variants (e.g. Standard -> High Flow),
the config write-back only stored the derived extruder map; every
per-variant filament value (retraction, nozzle temperature, flow,
flush...) kept the numbers resolved from the pre-slice static mapping.

Now both dynamic write-back sites (the by-layer branch and the
sequential stitch) branch on the result's dynamic support. Migrating
results run a mixed-filament expansion that regathers every
filament_options_with_variant key from the pristine per-variant
superset, giving a migrating filament one config slot per (extruder
type x nozzle volume type) it lands on - filament_self_index,
filament_extruder_variant, and all value arrays grow in lockstep - and
recompute the retract overrides with per-slot machine indices so a nil
slot falls back to its own variant's machine value. Non-migrating
dynamic results take the merged three-map write-back so re-applies
reproduce from the written maps. Unrouted filaments resolve from the
result's own default map, so slot resolution never depends on
filament_map round-tripping through the plate config.

Print::apply reproduces the identical expansion from the persisted
group result (shared dedupe helper, expansion function, and slot
indices on both sides): the expanded keys sit in the psWipeTower /
psGCodeExport invalidate lists, so without the reproduction every
re-apply after a selector slice would diff non-empty and permanently
invalidate. cal_non_support_filaments now resolves the extruder per
layer from the published result for dynamic groupings.

filament_map_2 keeps its apply-time static derivation; nothing on the
dynamic path reads it (the per-slot machine indices key the override
merge), and per-(extruder x volume-type) machine limits in the g-code
processor remain a documented follow-up.

Every change is gated behind is_dynamic_group_reorder() or a persisted
result with dynamic support; no profile sets the flag, so the static
fleet's instruction stream is unchanged (20/20 pinned-slice byte gate
identical, incl. the sequential repro sliced twice, deterministic).

Tests: expansion unit coverage (migrating slots, unrouted fallback via
the default map, mis-sized volume map ignored, nullable retract keys in
lockstep, slot machine index layout), an end-to-end stub-driven
write-back asserting expanded slots, per-layer config-index resolution,
the override merge incl. the nil-slot variant fallback, and re-apply
stability, plus a real selector slice staying valid across re-apply.
Suites green (libslic3r 48987/168, fff_print 633/60).

src/libslic3r/Print.hpp

@@ -1006,6 +1006,13 @@ public:
     const ToolOrdering& 		tool_ordering() const { return m_tool_ordering; }
 
     void update_filament_maps_to_config(std::vector<int> f_maps, std::vector<int> f_volume_maps = std::vector<int>{}, std::vector<int> f_nozzle_maps = std::vector<int>{});
+    // Write-back for a selector (per-layer planned) grouping result. When a filament actually
+    // migrates between nozzle variants, rebuilds the per-slot filament arrays so it holds one
+    // slot per variant and recomputes the extruder retract overrides against the expanded
+    // slots — update_filament_maps_to_config's single-slot rebuild cannot represent a
+    // migration. A result without migration reduces to a single grouping and takes the
+    // three-map write-back like the static paths.
+    void update_to_config_by_nozzle_group_result(const MultiNozzleUtils::LayeredNozzleGroupResult& group_result);
     void apply_config_for_render(const DynamicConfig &config);
 
     // 1 based group ids
@@ -1242,6 +1249,15 @@ private:
     void                _make_wipe_tower();
     void                finalize_first_layer_convex_hull();
     void                update_filament_self_index_cache();
+    // Deduplicates, per filament, the (extruder type x volume type) variants the grouping
+    // result routes it through; filaments the plan never routes get their default-map
+    // assignment so the slot resolution never depends on the (mutable) filament_map. config
+    // must carry extruder_type; returns false when it does not. Both the slice-time write-back
+    // and the apply-time reproduction call this with m_ori_full_print_config so the two
+    // expansions resolve identical slots.
+    bool                collect_filament_variant_uses(const MultiNozzleUtils::LayeredNozzleGroupResult& group_result,
+                                                      const DynamicPrintConfig& config,
+                                                      std::unordered_map<int, std::vector<FilamentVariantUse>>& uses) const;
 
     // Islands of objects and their supports extruded at the 1st layer.
     Polygons            first_layer_islands() const;