Merge branch 'tests/imex-coverage' into feedback

Brings in the IMEX test coverage (PA per-firmware, Temperature per-firmware,
[Variant] expansion, 3MF round-trip, imex_pem_tool_for helper + tests, and
the cherry-picked variant-truncation regression test).

Resolution notes:
- Two GCode.cpp call sites for set_pressure_advance had divergent edits:
    * tests/imex-coverage rewrote them to use the new imex_pem_tool_for
      helper (commit c2492ccc47), eliminating the inline parallel-mode
      check entirely.
    * feedback replaced the literal "primary" with kImexPrimaryMode in the
      same lines (commit 085f5ccec8).
  Resolution: keep the helper-call form. The kImex change is moot on lines
  the helper replaces, and imex_pem_tool_for in IMEXHelpers.cpp is also
  updated to use kImexPrimaryMode for consistency with the rest of the
  codebase.

- Test test_3mf.cpp updated for upstream's load_bbs_3mf signature change
  (PR adds is_orca_3mf out-parameter between is_bbl_3mf and file_version).
  All three call sites in the new IMEX 3MF round-trip tests pass &is_orca
  in addition to &is_bbl.

Full regression post-merge:
  libslic3r:    143 cases / 48,553 assertions  (+10 cases from new tests)
  fff_print:     24 cases /    245 assertions  (+10 cases from new tests)
  sla_print:     21 cases / 14,100 assertions
  libnest2d:     14 cases /    488 assertions
  slic3rutils:    3 cases /      3 assertions

All tests pass.

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

src/libslic3r/GCode.cpp

@@ -7644,13 +7644,7 @@ 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 != kImexPrimaryMode;
-            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
@@ -7950,11 +7944,7 @@ 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 != kImexPrimaryMode;
-        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