Extract Layer::choose_ironing_extruder for unit-testable ironing routing (#13467)

* Extract Layer::choose_ironing_extruder for unit-testable ironing routing

The ironing extruder selection in make_ironing() was a 5-line nested
conditional inlined at the top of the loop, with no isolated test
coverage. Pull the gating into a static helper so the routing decision
is unit-testable without spinning up the slicing pipeline.

Pure refactor: the helper preserves the original logic bit-for-bit
(NoIroning -> -1; AllSolid always enabled; TopSurfaces and TopmostOnly
require some top shells or, in spiral mode, more than one bottom shell;
TopmostOnly additionally requires being on the topmost layer; enabled
ironing routes to solid_infill_filament).

Add tests/fff_print/test_choose_ironing_extruder.cpp covering:
- AllSolid regardless of layer position
- TopSurfaces with top_shell_layers > 0
- TopSurfaces with top_shell_layers=0 + spiral mode + bottom_shell_layers>1
- TopmostOnly + topmost layer
- NoIroning short-circuit
- TopSurfaces with top_shell_layers=0 (and not spiral) -> disabled
- TopSurfaces, spiral, but bottom_shell_layers=1 -> disabled
- TopmostOnly on a non-topmost layer -> disabled

* Move ironing routing test into the Fill subsystem file

Rename the test to tests/libslic3r/test_fill.cpp and tag it [Fill] to
match the subsystem it covers, use flat behavioral test cases with
GENERATE for the parameterized ones, and drop the history narration from
the code comments.

* tests: move ironing routing tests into fff_print/test_fill.cpp

Keeps the Fill tests in one file, alongside the existing ironing
rotation-template test.

tests/fff_print/test_fill.cpp

@@ -15,6 +15,7 @@
 #include "libslic3r/Geometry.hpp"
 #include "libslic3r/Layer.hpp"
 #include "libslic3r/Print.hpp"
+#include "libslic3r/PrintConfig.hpp"
 #include "libslic3r/SVG.hpp"
 #include "libslic3r/libslic3r.h"
 
@@ -676,6 +677,73 @@ TEST_CASE("Ironing follows the solid infill rotation template", "[Fill]")
     REQUIRE(compared > int(ironing.size()) / 2);
 }
 
+
+namespace {
+
+PrintRegionConfig ironing_config(IroningType type,
+                                 int top_surface_filament_id = 1,
+                                 int top_shell_layers        = 3,
+                                 int bottom_shell_layers     = 1)
+{
+    PrintRegionConfig cfg;
+    cfg.ironing_type.value            = type;
+    cfg.top_surface_filament_id.value = top_surface_filament_id;
+    cfg.top_shell_layers.value        = top_shell_layers;
+    cfg.bottom_shell_layers.value     = bottom_shell_layers;
+    cfg.outer_wall_filament_id.value  = 1;
+    cfg.wall_loops.value              = 2;
+    return cfg;
+}
+
+} // namespace
+
+TEST_CASE("Ironing an all-solid region uses the top surface filament on every layer", "[Fill]")
+{
+    const PrintRegionConfig cfg = ironing_config(IroningType::AllSolid, /*top_surface_filament_id=*/2);
+    const bool is_topmost_layer = GENERATE(false, true);
+    CAPTURE(is_topmost_layer);
+    REQUIRE(Layer::choose_ironing_extruder(cfg, /*spiral_mode=*/false, is_topmost_layer) == 2);
+}
+
+TEST_CASE("Ironing top surfaces uses the top surface filament when the region has top shells", "[Fill]")
+{
+    const PrintRegionConfig cfg = ironing_config(IroningType::TopSurfaces,
+                                                 /*top_surface_filament_id=*/3,
+                                                 /*top_shell_layers=*/2);
+    REQUIRE(Layer::choose_ironing_extruder(cfg, /*spiral_mode=*/false, /*is_topmost_layer=*/false) == 3);
+}
+
+TEST_CASE("Ironing top surfaces without top shells needs spiral mode and more than one bottom shell", "[Fill]")
+{
+    const PrintRegionConfig one_bottom_shell = ironing_config(IroningType::TopSurfaces,
+                                                              /*top_surface_filament_id=*/1,
+                                                              /*top_shell_layers=*/0,
+                                                              /*bottom_shell_layers=*/1);
+    const PrintRegionConfig two_bottom_shells = ironing_config(IroningType::TopSurfaces,
+                                                               /*top_surface_filament_id=*/1,
+                                                               /*top_shell_layers=*/0,
+                                                               /*bottom_shell_layers=*/2);
+
+    REQUIRE(Layer::choose_ironing_extruder(two_bottom_shells, /*spiral_mode=*/true, /*is_topmost_layer=*/false) == 1);
+    REQUIRE(Layer::choose_ironing_extruder(one_bottom_shell, /*spiral_mode=*/true, /*is_topmost_layer=*/false) == -1);
+    REQUIRE(Layer::choose_ironing_extruder(two_bottom_shells, /*spiral_mode=*/false, /*is_topmost_layer=*/false) == -1);
+}
+
+TEST_CASE("Ironing the topmost surface only applies to the topmost layer", "[Fill]")
+{
+    const PrintRegionConfig cfg = ironing_config(IroningType::TopmostOnly, /*top_surface_filament_id=*/4);
+    REQUIRE(Layer::choose_ironing_extruder(cfg, /*spiral_mode=*/false, /*is_topmost_layer=*/true) == 4);
+    REQUIRE(Layer::choose_ironing_extruder(cfg, /*spiral_mode=*/false, /*is_topmost_layer=*/false) == -1);
+}
+
+TEST_CASE("A region with ironing turned off is never ironed", "[Fill]")
+{
+    const PrintRegionConfig cfg = ironing_config(IroningType::NoIroning);
+    const bool spiral_mode      = GENERATE(false, true);
+    CAPTURE(spiral_mode);
+    REQUIRE(Layer::choose_ironing_extruder(cfg, spiral_mode, /*is_topmost_layer=*/true) == -1);
+}
+
 TEST_CASE("Solid infill direction offsets every layer when no template is set", "[Fill]")
 {
     auto angles_for = [](int direction) {