* Fix CJK text garbled when font unavailable (#69)
Always use buildParagraph for text rendering when fonts are loaded,
even when the node's specific font isn't available. The paragraph
shaper falls back to the CJK font in fontFamilies, instead of the
previous drawText fallback which used Inter (no CJK glyphs).
* Add renderText unit tests for CJK font fallback behavior
* Add CJK visual regression test with Noto Sans SC fixture
- Download NotoSansSC-Regular.ttf as LFS-tracked test fixture
- Add setCJKFallbackFamily() export for headless/test use
- Visual test verifies CJK text renders through buildParagraph fallback
when node font is unavailable (the exact PR #89 scenario)
- Assert dark pixel count > 500 to distinguish real glyphs from tofu
