Reject OBJ UDIM faces spanning tiles

[Loggableim]

Summary

• detect OBJ faces whose vertices cross UDIM tile boundaries
• abort UDIM splitting with a localized error instead of continuing with an ambiguous tile
• add focused regression checks

Root cause

The OBJ splitter assigned each face to its average UV tile. Faces crossing tiles could corrupt split bookkeeping and crash the import.

Validation

• python paint/tests/test_issue_1870.py
• Local native project generation is currently blocked by the bundled amake stack-overflow error; GitHub platform builds will provide compile validation.

Fixes #1870