Refactor custom GCode handling and mixed filament management

- Updated `custom_tool_changes` function to use filament IDs instead of extruder IDs, ensuring better compatibility with mixed filament configurations.
- Enhanced `MixedFilament` structure to include a `stable_id` for persistent identity across mixed filament entries, improving remapping during updates.
- Introduced new methods for stable ID allocation and normalization in `MixedFilamentManager`.
- Updated serialization and loading functions to maintain stable IDs, ensuring consistent behavior when mixed filament entries are modified.
- Added tests to verify the correct handling of tool changes and stable ID remapping in mixed filament scenarios.

tests/libslic3r/test_custom_gcode.cpp

@@ -0,0 +1,28 @@
+#include <catch2/catch.hpp>
+
+#include "libslic3r/CustomGCode.hpp"
+
+using namespace Slic3r;
+
+TEST_CASE("Custom layer tool changes keep mixed virtual filament ids", "[CustomGCode]")
+{
+    CustomGCode::Info info;
+    info.gcodes.emplace_back(CustomGCode::Item{1.25, CustomGCode::ToolChange, 5, "", ""});
+
+    const auto tool_changes = CustomGCode::custom_tool_changes(info, 6);
+
+    REQUIRE(tool_changes.size() == 1);
+    CHECK(tool_changes.front().first == Approx(1.25));
+    CHECK(tool_changes.front().second == 5u);
+}
+
+TEST_CASE("Custom layer tool changes still clamp stale filament ids", "[CustomGCode]")
+{
+    CustomGCode::Info info;
+    info.gcodes.emplace_back(CustomGCode::Item{2.0, CustomGCode::ToolChange, 7, "", ""});
+
+    const auto tool_changes = CustomGCode::custom_tool_changes(info, 6);
+
+    REQUIRE(tool_changes.size() == 1);
+    CHECK(tool_changes.front().second == 1u);
+}