refactor(imex): extract imex_pem_tool_for helper + unit tests

The physical_extruder_map translation used by IMEX per-tool PA emission was
inlined identically at two sites in GCode.cpp (tool-change and second-layer
transition). Extract to IMEXHelpers so the routing rule ("parallel mode AND
populated pem → physical index, else -1") is testable in isolation and the
call sites read as intent rather than re-deriving the conditional.

Production change is behavior-preserving:
- Same predicate (`!mode.empty() && mode != "primary"`)
- Same empty-pem short-circuit returning -1
- Same get_at() dispatch on hit
- Both call sites replaced with a single call

Four unit tests in [IMEX] cover the routing matrix:
  - non-IMEX ("") and primary mode short-circuit
  - parallel mode + empty pem short-circuits (defense-in-depth; get_at would
    throw on empty values otherwise)
  - identity pem (non-MMU IDEX) routes filament to itself
  - MMU collapse routes multiple logical slots to one physical (7-slot profile
    with 4-lane MMU on physical 0 and direct drives on 1/2/3)

All IMEX + Variant regression suites pass post-refactor (133 assertions / 48
cases under libslic3r, 25 assertions / 6 cases under fff_print [PressureAdvance]).

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

src/libslic3r/GCode.cpp

@@ -7589,13 +7589,7 @@ std::string GCode::set_extruder(unsigned int new_filament_id, double print_z, bo
             // In IMEX parallel modes each carriage needs an explicit tool address.
             // In primary mode (single active tool) regular tool changes handle PA
             // so no qualifier is needed — same as a non-IMEX printer.
-            // Guard the pem lookup: PrintApply populates pem when printer_extruder_id
-            // is set, but defense-in-depth prevents a throw from get_at on any
-            // empty-pem path that might slip through in exotic profiles.
-            const bool imex_parallel = !m_imex_parallel_mode.empty() && m_imex_parallel_mode != "primary";
-            const int pa_tool = (imex_parallel && !m_config.physical_extruder_map.values.empty())
-                ? m_config.physical_extruder_map.get_at((int)new_filament_id)
-                : -1;
+            const int pa_tool = imex_pem_tool_for((int)new_filament_id, m_imex_parallel_mode, m_config.physical_extruder_map);
             gcode += m_writer.set_pressure_advance(m_config.pressure_advance.get_at(new_filament_id), pa_tool);
             // Orca: Adaptive PA
             // Reset Adaptive PA processor last PA value
@@ -7895,11 +7889,7 @@ std::string GCode::set_extruder(unsigned int new_filament_id, double print_z, bo
         gcode += m_ooze_prevention.post_toolchange(*this);
 
     if (m_config.enable_pressure_advance.get_at(new_filament_id)) {
-        // Empty-pem guard mirrors the earlier PA site; get_at throws on empty values.
-        const bool imex_parallel = !m_imex_parallel_mode.empty() && m_imex_parallel_mode != "primary";
-        const int pa_tool = (imex_parallel && !m_config.physical_extruder_map.values.empty())
-            ? m_config.physical_extruder_map.get_at((int)new_filament_id)
-            : -1;
+        const int pa_tool = imex_pem_tool_for((int)new_filament_id, m_imex_parallel_mode, m_config.physical_extruder_map);
         gcode += m_writer.set_pressure_advance(m_config.pressure_advance.get_at(new_filament_id), pa_tool);
         // Orca: Adaptive PA
         // Reset Adaptive PA processor last PA value

src/libslic3r/IMEXHelpers.cpp

@@ -33,6 +33,14 @@ ConfigOptionInts effective_physical_extruder_map(const PresetBundle& pb)
     return effective_physical_extruder_map(explicit_pem, pei);
 }
 
+int imex_pem_tool_for(int filament_id, const std::string& parallel_mode, const ConfigOptionInts& pem)
+{
+    const bool imex_parallel = !parallel_mode.empty() && parallel_mode != "primary";
+    if (!imex_parallel || pem.values.empty())
+        return -1;
+    return pem.get_at(filament_id);
+}
+
 int first_filament_for_physical_head(const ConfigOptionInts& pem, int physical)
 {
     const auto& v = pem.values;

src/libslic3r/IMEXHelpers.hpp

@@ -30,6 +30,14 @@ ConfigOptionInts effective_physical_extruder_map(const ConfigOptionInts* explici
 // all agree on what the slicer will see.
 ConfigOptionInts effective_physical_extruder_map(const PresetBundle& pb);
 
+// Returns the physical extruder index to emit as a per-tool qualifier (PA / temperature)
+// for the given filament slot, or -1 when the current IMEX state does not warrant a
+// per-tool qualification: non-IMEX / primary mode, or the pem is empty / unpopulated.
+// Used at tool-change and second-layer transition call sites where IMEX parallel modes
+// route emission through the physical extruder, while ordinary tool changes emit bare
+// firmware commands like any non-IMEX printer.
+int imex_pem_tool_for(int filament_id, const std::string& parallel_mode, const ConfigOptionInts& pem);
+
 // Returns the lowest 0-based logical filament index L such that pem[L] == physical.
 // Returns -1 if no filament routes to `physical`.
 // Degenerate case: an empty pem returns 0 when `physical == 0` (identity-on-head-0