* 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.
Daniel Williams