feat(imex): plate ghost renderer + per-plate filament map

Replaces the plater-icon popover with colored transparent ghost copies
of primary-head instances on the plate, one per secondary active head
under its Copy/Mirror role transform. Left-click on a ghost opens a
compact filament picker popover for the ghost's head (MMU lane override).
Ghosts track the primary through drag/rotate/scale/mirror and invalidate
on mode or pem changes.

Key pieces:
  IMEXHelpers -- imex_head_transform (Primary/Copy/Mirror), shared role
    parser, per-head filament resolution with X-axis Mirror anchor.
  PartPlate -- ghost state, volume rebuild on mode/map/object mutation,
    primary_origin plumbed for Mirror reflection across the primary-row
    gantry plane.
  GLCanvas3D -- ghost rendering with per-head filament color and
    translucent blending; picking routed via volume composite id.
  Plater -- ghost click + tooltip; plater icon left-click always cycles.
  IMEXFilamentPickerPopover -- BitmapComboBox row for one secondary head,
    writes imex_head_filament_map on selection.
  bbs_3mf -- round-trip the per-plate imex_head_filament_map option.
  PrintConfig -- add imex_head_filament_map as a plate option.

MMU/AFC routing for parallel modes relies on the printer profile's
physical_extruder_map (see prior commit for authoring format). Primary-
row heads and their per-plate filament overrides are resolved through
that map, so PA and temperature emission address the correct physical
extruder when multiple logical slots share one carriage.

Tests: IMEXHelpers coverage for Primary/Copy/Mirror transforms
including a 2x2 off-row regression guard for the X-axis reflection fix.

src/libslic3r/PrintConfig.cpp

@@ -5555,6 +5555,13 @@ void PrintConfigDef::init_fff_params()
     def->mode = comAdvanced;
     def->set_default_value(new ConfigOptionString("primary"));
 
+    def = this->add("imex_head_filament_map", coString);
+    def->label = L("IMEX head filament map");
+    def->tooltip = L("Per-plate override mapping physical heads to filament slots "
+                     "(1-based). Empty means use pem-inversion defaults.");
+    def->mode = comDevelop;
+    def->set_default_value(new ConfigOptionString(""));
+
     def = this->add("imex_mode_names", coStrings);
     def->label = L("IDEX/IQEX Mode Names");
     def->tooltip = L("Display names for each user-defined IDEX/IQEX parallel print mode.");