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/PresetBundle.cpp

@@ -49,6 +49,14 @@ static std::vector<std::string> s_project_options {
     "nozzle_volume_type",
     "filament_map_mode",
     "filament_map"
+    // physical_extruder_map intentionally NOT here: it's owned by the printer
+    // preset (s_Preset_printer_options). Listing it project-scoped caused
+    // project_config's default [0] to clobber the preset's authored value
+    // (e.g. AFC-shaped [0,1,1,1,1]) during full_fff_config() merge, and the
+    // clobbered value then rode into saved 3mfs and back into the edited
+    // preset on reload. MoonrakerPrinterAgent's runtime set_key_value still
+    // works — it creates the option on project_config on demand without
+    // needing it pre-initialized here.
 };
 
 //Orca: add custom as default