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/slic3r/GUI/IMEXFilamentPickerPopover.cpp

@@ -0,0 +1,124 @@
+#include "slic3r/GUI/IMEXFilamentPickerPopover.hpp"
+
+#include <wx/stattext.h>
+#include <wx/dcmemory.h>
+#include <wx/image.h>
+
+#include "libslic3r/IMEXHelpers.hpp"
+#include "libslic3r/PresetBundle.hpp"
+#include "slic3r/GUI/BitmapComboBox.hpp"
+#include "slic3r/GUI/PartPlate.hpp"
+#include "slic3r/GUI/GUI_App.hpp"
+#include "slic3r/GUI/wxExtensions.hpp"
+
+namespace Slic3r { namespace GUI {
+
+IMEXFilamentPickerPopover::IMEXFilamentPickerPopover(
+    wxWindow* parent, PartPlate* plate,
+    const ConfigOptionInts& pem, int physical_head,
+    CommitCallback on_commit)
+    : wxPopupTransientWindow(parent, wxBORDER_SIMPLE)
+    , m_plate(plate)
+    , m_pem(pem)
+    , m_physical_head(physical_head)
+    , m_on_commit(std::move(on_commit))
+{
+    m_root_sizer = new wxBoxSizer(wxHORIZONTAL);
+    SetSizer(m_root_sizer);
+
+    build_row();
+
+    m_root_sizer->SetSizeHints(this);
+    SetClientSize(m_root_sizer->ComputeFittingClientSize(this));
+    Layout();
+}
+
+void IMEXFilamentPickerPopover::popup_at_cursor()
+{
+    Position(wxGetMousePosition(), wxSize(0, 0));
+    Popup();
+}
+
+void IMEXFilamentPickerPopover::build_row()
+{
+    m_root_sizer->Clear(true);
+
+    m_root_sizer->AddSpacer(10);
+
+    auto* label = new wxStaticText(this, wxID_ANY,
+            wxString::Format("T%d", m_physical_head));
+    m_root_sizer->Add(label, 0, wxALIGN_CENTER_VERTICAL | wxTOP | wxBOTTOM, 6);
+
+    m_root_sizer->AddSpacer(10);
+
+    std::vector<int> lane_logicals;
+    for (size_t L = 0; L < m_pem.values.size(); ++L)
+        if (m_pem.values[L] == m_physical_head)
+            lane_logicals.push_back((int)L);
+
+    std::vector<wxBitmap*> bmps = get_extruder_color_icons(false /*thin_icon*/);
+
+    auto* choice = new BitmapComboBox(this, wxID_ANY, wxEmptyString,
+                                      wxDefaultPosition, wxSize(140, -1),
+                                      0, nullptr, wxCB_READONLY);
+    const int left_pad_px = 8;
+    auto pad_left = [&](const wxBitmap& src) -> wxBitmap {
+        if (!src.IsOk()) return src;
+        wxImage im(src.GetWidth() + left_pad_px, src.GetHeight());
+        im.InitAlpha();
+        unsigned char* a = im.GetAlpha();
+        std::fill(a, a + im.GetWidth() * im.GetHeight(), (unsigned char)0);
+        wxBitmap out(im);
+        wxMemoryDC dc(out);
+        dc.DrawBitmap(src, left_pad_px, 0, true);
+        dc.SelectObject(wxNullBitmap);
+        return out;
+    };
+
+    for (int L : lane_logicals) {
+        const wxString label = wxString::Format("filament %d", L + 1);
+        if (L >= 0 && L < (int)bmps.size() && bmps[L] && bmps[L]->IsOk())
+            choice->Append(label, pad_left(*bmps[L]));
+        else
+            choice->Append(label, wxNullBitmap);
+    }
+
+    const auto map = m_plate->get_imex_head_filament_map();
+    auto it = map.find(m_physical_head);
+    int selected_slot_1based = (it != map.end())
+        ? it->second
+        : (lane_logicals.empty() ? -1 : lane_logicals.front() + 1);
+
+    for (int i = 0; i < (int)lane_logicals.size(); ++i) {
+        if (lane_logicals[i] + 1 == selected_slot_1based) {
+            choice->SetSelection(i);
+            break;
+        }
+    }
+
+    choice->Bind(wxEVT_COMBOBOX, [this, lane_logicals, choice](wxCommandEvent&) {
+        int sel = choice->GetSelection();
+        if (sel < 0 || sel >= (int)lane_logicals.size()) return;
+        on_filament_selected(lane_logicals[sel] + 1);
+        Dismiss();
+    });
+
+    m_root_sizer->Add(choice, 0, wxALIGN_CENTER_VERTICAL | wxALL, 6);
+    m_root_sizer->AddSpacer(4);
+}
+
+void IMEXFilamentPickerPopover::on_filament_selected(int slot_1_based)
+{
+    auto map = m_plate->get_imex_head_filament_map();
+    // Erase when the user reselects the pem default for this head, so the 3mf
+    // doesn't carry no-op overrides (same invariant as PR3b).
+    const int default_logical = first_filament_for_physical_head(m_pem, m_physical_head);
+    if (default_logical >= 0 && slot_1_based == default_logical + 1)
+        map.erase(m_physical_head);
+    else
+        map[m_physical_head] = slot_1_based;
+    m_plate->set_imex_head_filament_map(map);
+    if (m_on_commit) m_on_commit();
+}
+
+}} // namespace Slic3r::GUI