feat(renderer): powerline + block char pixel-perfect rendering (#22)

- Add renderBlockChar() for U+2580-U+259F (block elements) as filled
  rectangles, eliminating inter-character gaps in ASCII art/progress bars
- Add renderPowerlineGlyph() for U+E0B0-U+E0B7 as canvas vector paths,
  ensuring glyphs span exactly the cell height regardless of font
- Switch measureFont() to fontBoundingBox metrics so cell height
  accommodates the full font cap-height (required by powerline chars)
- Merge DPR-aware rounding from PR coder#146 with fontBoundingBox: cells
  stay pixel-perfect at non-integer device pixel ratios
- Pass cursor.style from IRenderable.getCursor() through to renderCursor()
  so callers can override the cursor shape
- buildFontString() helper quotes font family names that contain spaces
- Add demo/bin/render-test.ts and demo/render-test.html for visual
  regression testing (puppeteer auto-installed on demand)
- Fix pre-existing biome lint issues in viewport/iris test files


Inspired-by: coder#128

Co-authored-by: Stuart Lang <stuart.b.lang@gmail.com>

Author: diegosouzapw