feat: letterSpacing option in prepare()

[ImpulseB23]

Closes #78

Adds letterSpacing (in px) to PrepareOptions. During prepare(), each segment's cached width is adjusted by graphemeCount * letterSpacing, with trailing letter-spacing trimmed at line boundaries via lineEndFitAdvance. The layout() hot path is untouched.

Handles prefix currencies, CJK, pre-wrap, negative spacing (tighter tracking), and overflow-wrap grapheme-level breaking. Grapheme counting uses Intl.Segmenter and is gated behind letterSpacing !== 0 so there's zero overhead on the default path.

const prepared = prepare(text, font, { letterSpacing: 2 })
const { height } = layout(prepared, width, lineHeight)

Tested in Chromium on Windows across various texts, widths, and spacing values (0-10px). All match browser line counts. 87/87 tests pass including 7 new letter-spacing tests.

[Copilot]

Pull request overview

Adds an optional letterSpacing (px) to prepare() so cached segment widths can account for CSS-like tracking without changing the layout() API surface.

Changes:

• Extend PrepareOptions with letterSpacing?: number and thread it through prepareInternal() -> measureAnalysis().
• Adjust measured segment widths (and overflow-wrap grapheme widths/prefix widths) to include letter-spacing effects.
• Add unit tests covering basic width deltas, zero/negative spacing, and a few wrapping scenarios.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/layout.ts	Implements letterSpacing during prepare-time measurement by adjusting cached widths and line-end advances.
src/layout.test.ts	Adds tests for width deltas and basic wrapping behavior when letterSpacing is set.

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