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.

src/libslic3r/Layer.hpp

@@ -16,6 +16,7 @@ using LayerPtrs = std::vector<Layer*>;
 class LayerRegion;
 using LayerRegionPtrs = std::vector<LayerRegion*>;
 class PrintRegion;
+class PrintRegionConfig;
 class PrintObject;
 class Print;
 
@@ -200,6 +201,11 @@ public:
                                                                            FillAdaptive::Octree *support_fill_octree,
                                                                            FillLightning::Generator* lightning_generator) const;
     void 					make_ironing();
+    // Returns the filament id (1-based) the region is ironed with, or -1 when the
+    // region is not ironed.
+    static int              choose_ironing_extruder(const PrintRegionConfig &cfg,
+                                                    bool spiral_mode,
+                                                    bool is_topmost_layer);
     void                    make_contour_z(const sla::IndexedMesh &mesh);
 
     void                    export_region_slices_to_svg(const char *path) const;