Expose fontKerning option in prepare() — align measurement with rendering

## Problem

pretext's core value is "accurate measurement without DOM reflow". However, measurement is always locked to the browser default kerning (`"auto"`, which typically applies kerning) because `prepare()` never sets `ctx.fontKerning` when measuring with `measureText()`.

When a consumer renders with kerning disabled (`font-kerning: none` / `ctx.fontKerning = "none"`), pretext's measured widths no longer match the rendered text, causing bounding-box / layout mismatches.

So this isn't really "I want kerning off" — it's that there's **no way to align the measurement's kerning mode with the actual rendering**. Today `PrepareOptions` only exposes `whiteSpace`, `wordBreak`, and `letterSpacing`.

## Proposal

Add an optional `fontKerning` to `PrepareOptions`:

```ts
export type PrepareOptions = {
whiteSpace?: WhiteSpaceMode
wordBreak?: WordBreakMode
letterSpacing?: number
fontKerning?: 'auto' | 'normal' | 'none' // default 'auto' (current behavior)
}
```

Then set `ctx.fontKerning` alongside `ctx.font` in the measurement path.

Defaulting to `'auto'` keeps the current behavior intact. Users who render with kerning are unaffected; users who render with a different kerning mode can align measurement with their rendering. → **non-breaking**.

### Note on caching
Segment widths are cached by `(segment, font)`. Since kerning mode affects width, the cache key needs to include `fontKerning` (e.g. `(segment, font, fontKerning)`) so a width measured under a different kerning mode isn't reused incorrectly.

Happy to open a PR if this direction sounds good.

Start at prepare() and its PrepareOptions type, then trace the measurement path where ctx.font is set before measureText(). Inspect the segment-width cache keyed by segment and font. Done means the optional fontKerning setting preserves the default behavior, affects measurement, and prevents widths from being reused across kerning modes.

Written by the indexing model from the issue text.