test: add pytest.importorskip for cairosvg-dependent tests

Gracefully skips figment tests when system Cairo library is unavailable
instead of crashing with opaque OSError during test collection.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_figment_overlay.py

@@ -1,5 +1,9 @@
 """Tests for render_figment_overlay in engine.layers."""
 
+import pytest
+
+pytest.importorskip("cairosvg", reason="cairosvg requires system Cairo library")
+
 from effects_plugins.figment import FigmentPhase, FigmentState
 from engine.layers import render_figment_overlay