feat: add powerline and block character rendering, visual test suite

- Implement renderBlockChar() for U+2580-U+259F block elements as pixel-perfect rectangles
- Implement renderPowerlineGlyph() for U+E0B0-U+E0BF glyphs as vector shapes
- Update font metrics to use fontBoundingBox for accurate cell height
- Add buildFontString() helper for proper font family quoting
- Add cursor style support from buffer and cursorAccent color rendering
- Add 15 visual regression tests covering text, colors, cursors, wide chars, and special glyphs
- Include headless Puppeteer test runner for CI/CD integration
- Add baseline PNG images for all tests passing

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: slang25

.gitignore

@@ -6,3 +6,6 @@ dist/
 
 # WASM build output (built locally and in CI)
 ghostty-vt.wasm
+
+# Visual render test failure artifacts
+demo/baselines/*.fail.png