feat(slicing): enforce filament flow-type restrictions in auto grouping

Filament grouping already consumed per-filament forbidden nozzle volume
types, but every call site passed an empty map, so a variant-restricted
filament (e.g. one limited to "Direct Drive TPU High Flow") could be
auto-grouped onto an incompatible nozzle flow type on multi-variant
printers.

- add convert_to_nvt_type() to parse extruder variant strings
- add Print::get_filament_unprintable_flow(): forbidden volume types =
  printer extruder variants minus the filament's declared variants;
  filaments declaring no variants stay unrestricted
- feed the map into grouping at the by-object path (Print.cpp) and all
  six mapping/planning sites in reorder_extruders_for_minimum_flush_volume
- unit-test the string parser

Non-restricted configurations produce an empty map, so existing
printers' grouping and g-code are unchanged.

tests/libslic3r/CMakeLists.txt

@@ -21,6 +21,7 @@ add_executable(${_TEST_NAME}_tests
     test_polygon.cpp
     test_mutable_polygon.cpp
     test_mutable_priority_queue.cpp
+    test_nozzle_volume_type.cpp
     test_stl.cpp
     test_meshboolean.cpp
     test_marchingsquares.cpp

tests/libslic3r/test_nozzle_volume_type.cpp

@@ -0,0 +1,31 @@
+#include <catch2/catch_all.hpp>
+
+#include "libslic3r/PrintConfig.hpp"
+
+using namespace Slic3r;
+
+TEST_CASE("convert_to_nvt_type maps extruder variant strings to nozzle volume types", "[Config]")
+{
+    SECTION("Direct Drive variants") {
+        REQUIRE(convert_to_nvt_type("Direct Drive Standard") == nvtStandard);
+        REQUIRE(convert_to_nvt_type("Direct Drive High Flow") == nvtHighFlow);
+        REQUIRE(convert_to_nvt_type("Direct Drive TPU High Flow") == nvtTPUHighFlow);
+    }
+
+    SECTION("Bowden variants") {
+        REQUIRE(convert_to_nvt_type("Bowden Standard") == nvtStandard);
+        REQUIRE(convert_to_nvt_type("Bowden High Flow") == nvtHighFlow);
+    }
+
+    SECTION("Unparsable strings fall back to hybrid") {
+        REQUIRE(convert_to_nvt_type("Unknown Extruder") == nvtHybrid);
+        REQUIRE(convert_to_nvt_type("") == nvtHybrid);
+        REQUIRE(convert_to_nvt_type("High Flow") == nvtHybrid);
+        REQUIRE(convert_to_nvt_type("Direct Drive") == nvtHybrid);
+    }
+
+    SECTION("Whitespace around the volume-type remainder is trimmed") {
+        REQUIRE(convert_to_nvt_type("Direct Drive  High Flow ") == nvtHighFlow);
+        REQUIRE(convert_to_nvt_type(" Bowden Standard") == nvtStandard);
+    }
+}