Rewrite Line.svelte to render leftText / windowChars / rightText regions
from a LineRenderModel. Bulk regions render as native shaped text runs so
the browser applies kerning and ligatures; per-char DOM is reserved for
the N-char crossfade window straddling the slider.
Slim Character.svelte: drop the unused proximity prop and the redundant
font-size/font-weight/letter-spacing styles now inherited from the line
container.
Switch SliderArea.svelte to instantiate DualFontLayout and derive each
line's render model via computeLineRenderModel(line, sliderPos,
containerWidth, WINDOW_SIZE).
Ilia Mashkov
6153769317