feat(imex): effective_physical_extruder_map helper + diagnostic no-filament tooltip

Centralize the project→printer→printer_extruder_id fallback for the physical
extruder map. PrintApply, PartPlate (ghost color + cache key), Plater (tooltip
+ click gate) all previously open-coded the three-step lookup, and each handled
the "pem unset, derive from pei" case slightly differently — an IDEX printer
without an explicit pem could paint an UNPRINTABLE_COLOR ghost even though the
slicer would have derived a valid mapping.

- IMEXHelpers: add effective_physical_extruder_map(explicit_pem, pei) and a
  PresetBundle overload that wraps the project→printer precedence.
- PrintApply: use the helper in place of the inline pei→pem normalization.
- PartPlate / Plater: call the PresetBundle overload at every ghost-color,
  ghost-cache-key, tooltip, and click-gate site.
- Plater::format_imex_ghost_tooltip: when no filament resolves to a head,
  surface an actionable message directing the user to extend the extruder
  count in the Machine tab, instead of the generic "(no filament routed)".
- IMEXFilamentPickerPopover: hold m_pem by value so callers can pass a
  stack-local derived pem without lifetime worries.
- Tests: 5 new cases covering explicit-wins, default-pem fallback, null
  inputs, and the IDEX ghost-color regression that motivated this.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/libslic3r/IMEXHelpers.cpp

@@ -6,8 +6,33 @@
 #include <sstream>
 #include <unordered_set>
 
+#include "libslic3r/PresetBundle.hpp"
+
 namespace Slic3r {
 
+ConfigOptionInts effective_physical_extruder_map(const ConfigOptionInts* explicit_pem,
+                                                  const ConfigOptionInts* printer_extruder_id)
+{
+    if (explicit_pem && explicit_pem->values.size() >= 2)
+        return *explicit_pem;
+    ConfigOptionInts derived;
+    if (printer_extruder_id) {
+        derived.values.reserve(printer_extruder_id->values.size());
+        for (int v : printer_extruder_id->values)
+            derived.values.push_back(v - 1);
+    }
+    return derived;
+}
+
+ConfigOptionInts effective_physical_extruder_map(const PresetBundle& pb)
+{
+    const ConfigOptionInts* explicit_pem = pb.project_config.option<ConfigOptionInts>("physical_extruder_map");
+    if (!explicit_pem || explicit_pem->values.size() < 2)
+        explicit_pem = pb.printers.get_edited_preset().config.option<ConfigOptionInts>("physical_extruder_map");
+    const ConfigOptionInts* pei = pb.printers.get_edited_preset().config.option<ConfigOptionInts>("printer_extruder_id");
+    return effective_physical_extruder_map(explicit_pem, pei);
+}
+
 int first_filament_for_physical_head(const ConfigOptionInts& pem, int physical)
 {
     const auto& v = pem.values;