feat(engine): resolve per-variant config columns for raw filament reads on the export path

When a per-layer nozzle grouping migrates a filament across nozzle
variants, the write-back turns two groups of config arrays from
filament-indexed into column-indexed: the per-variant filament options
(one column per variant a filament uses) and the merged extruder
retract overrides (resized to the column count by apply_override).
Export-path readers that still indexed them with the raw filament id
read a neighbor's column for every filament ordered after a migrating
one: toolchange/standby temperatures (M104/M109), retraction lengths
and feedrates, wipe distance, z-hop types, air-filtration keys, and -
through the Extruder's cached flow term - the extrusion E of every
move.

Now every such read resolves its column through the existing
layer-aware resolver (get_filament_config_index ->
Print::get_filament_config_indx), which returns the raw filament id
whenever no per-layer grouping result is published, so static prints
are byte-inert by construction. The Extruder itself has no layer
knowledge, so it gains an injected config column (set_config_index,
default = filament id) that the generator refreshes at the only two
resolution-changing events - layer change and writer toolchange - and
that re-syncs the cached e_per_mm3 flow term. Old-filament reads
resolve at the current layer, which is safe because the per-layer maps
are gap-filled carry-forward. Whole-array placeholder copies
(toolchange temperature overrides) are rebuilt in filament order,
mirroring the existing per-variant placeholder remap. The resolvers
move to the public section so non-friend helpers (ooze prevention) can
resolve too.

Documented, deliberately unchanged: the wipe tower's per-filament
parameter rows (no layer dimension; tower x per-layer grouping is a
follow-up), travel_slope's physical-extruder read, estimator pre-heat
bookkeeping temps, and index-0 header diagnostics.

Verification: new Extruder column-injection scenario (defaults, column
follow + flow-cache rescale, filament-indexed reads unaffected, reset
semantics) and a migrating write-back case proving the column shift for
filaments ordered after a migrator and the resolver tracking it (11 +
14 assertions); suites green (libslic3r 48998/169, fff_print 655/61);
20/20 pinned-slice byte gate bit-identical (incl. sequential repro x2
deterministic).

src/libslic3r/GCode/WipeTower2.cpp

@@ -1333,6 +1333,10 @@ void WipeTower2::set_extruder(size_t idx, const PrintConfig& config)
     //while (m_filpar.size() < idx+1)   // makes sure the required element is in the vector
     m_filpar.push_back(FilamentParameters());
 
+    // Orca: one row per filament, indexed by the raw filament id. Under a per-layer nozzle
+    // grouping the per-variant arrays may hold several columns per filament; the tower has no
+    // layer dimension here, so it keeps the filament's first column (tower x per-layer
+    // grouping is a documented follow-up).
     m_filpar[idx].material = config.filament_type.get_at(idx);
     if (m_wipe_tower_filament > 0)
         m_filpar[idx].is_soluble = (idx != size_t(m_wipe_tower_filament - 1));