[davidbrochart]

Adjacent Unicode block glyphs can show seams when drawn with fillText(), depending on font metrics and display scaling. This is visible in the solid blue parts of the LazyVim dashboard logo, even after aligning cell backgrounds.

Render solid Unicode Block Elements as geometric rectangles with shared physical-pixel boundaries: full blocks, fractional horizontal/vertical blocks, and quadrant combinations. Preserve the existing foreground selection, inverse, faint, cursor-color, and invisible handling. Ordinary text, grapheme clusters, shaded blocks, and box-drawing glyphs retain font rendering.

This PR is independent of #211: that change fixes cell backgrounds; this fixes block-character foregrounds. It does not attempt to fix seams in box-drawing font glyphs.

Validation:

• 27 new tests cover block and quadrant geometry, shared edges at device-pixel ratios 1, 1.1, 1.25, 1.5, and 2, colors, and font fallback.
• A separate real-canvas pixel check verifies uniform foreground coverage across adjacent full blocks at all five scales.
• Full suite: 399 passed, 0 failed.
• Formatting, lint, TypeScript checks, and full WASM/JavaScript build pass.