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/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;