FIX: avoid debug asserts in multifilament project loading

[altendky]

Summary

• Fix a debug-build assertion when loading by-object multifilament projects that use filament slot 1.
• Treat PrintObject::object_extruders() values as 0-based filament indexes in by-object multifilament validation.

Details

PrintObject::object_extruders() returns 0-based filament indexes, so 0 is valid and represents the first filament. The by-object branch in Print::check_multi_filament_valid() incorrectly asserted that each returned extruder index was greater than zero, causing debug builds to abort while loading valid multifilament .3mf projects.

The fix removes that invalid assertion and uses an unsigned local set matching the returned index type.

Testing

• git diff --check master...HEAD

Notes

• No regression test was added; the existing FFF print test target is currently stale and fails to build on unrelated API drift.