fff_print: a maintainable testing framework (proposal + coverage) (#14426)

* fix: initialize Print::m_isBBLPrinter

Built outside the GUI/CLI (headless tests, embedded use) the member was read
uninitialized: is_BBL_printer()/wipe_tower_type() feed it into ToolOrdering,
which then non-deterministically dropped per-feature filament assignments.
Default it to false, the value the GUI and CLI already assign for non-Bambu
printers.

* docs(test): add the fff_print testing contract

tests/fff_print/README.md codifies how the suite is organized: one file per
subsystem (each owning both in-memory and emitted-G-code assertions), flat
behavioral test names with a single [Subsystem] tag, a robust-tests guide,
the shared helpers, and an add-a-test checklist. Linked from tests/CLAUDE.md.

* test(fff_print): reorganize the suite to the contract and add coverage

Bring every subsystem into one file per the README: rename the test_data
harness to test_helpers; consolidate skirt/brim; split multi-filament and
cooling into their own files; disperse the test_printgcode grab-bag and the
end-to-end smoke scenario into focused tests; fold test_gcode into
test_gcodewriter. Standardize names and tags, align cube tests on the cube()
helper, and de-qualify the flagship files.

New coverage: multi-filament per-feature and per-object routing; a skirt/brim
behavior matrix (the #14333 rework, including brim ears, with regression
coverage for #14319 and #14366); resolved extrusion-width and config
comments; custom-G-code placeholders; fan control and speed-marker
consumption.

Re-enable three slice tests previously tagged [NotWorking]: the clipper
"Coordinate outside allowed range" error that disabled them was specific to a
past CI runner environment and no longer reproduces.

* test(fff_print): tag arm64-flaky skirt/brim tests NotWorking

Four skirt/brim slice tests intermittently throw ClipperLib's "Coordinate
outside allowed range" on the macOS and Windows arm64 CI toolchains (an FP
divergence, not a slicing bug; see PR #14207). Linux x86_64 and aarch64 are
unaffected. Tag them [NotWorking] so ctest -LE NotWorking skips them.

* test(fff_print): re-enable the arm64 skirt/brim tests

These were tagged [NotWorking] as a stopgap when myfork's daily-driver build
combined them with the cross-platform CI on a base that predated upstream's
m_origin fix (99dea01cc3). With upstream merged in, Print::m_origin is
initialized and the "Coordinate outside allowed range" throw is gone, so the
tests pass on macOS/Windows arm64. Drop the tags.

tests/fff_print/test_fill.cpp

@@ -11,14 +11,14 @@
 #include "libslic3r/SVG.hpp"
 #include "libslic3r/libslic3r.h"
 
-#include "test_data.hpp"
+#include "test_helpers.hpp"
 
 using namespace Slic3r;
 
 bool test_if_solid_surface_filled(const ExPolygon& expolygon, double flow_spacing, double angle = 0, double density = 1.0);
 
 #if 0
-TEST_CASE("Fill: adjusted solid distance") {
+TEST_CASE("Adjusted solid distance", "[Fill]") {
     int surface_width = 250;
     int distance = Slic3r::Flow::solid_spacing(surface_width, 47);
     REQUIRE(distance == Catch::Approx(50));
@@ -26,7 +26,7 @@ TEST_CASE("Fill: adjusted solid distance") {
 }
 #endif
 
-TEST_CASE("Fill: Pattern Path Length", "[Fill]") {
+TEST_CASE("Pattern path length", "[Fill]") {
     std::unique_ptr<Slic3r::Fill> filler(Slic3r::Fill::new_from_type("rectilinear"));
     filler->angle = float(-(PI)/2.0);
 	FillParams fill_params;