feat: expose fontKerning option in prepare()

[alexiskowalski]

Summary

Fixes #199 by letting prepare() / prepareWithSegments() set canvas fontKerning while measuring.

Pretext's value is accurate measurement without DOM reflow, but measurement was always locked to the browser default kerning (auto). Consumers that paint with font-kerning: none / ctx.fontKerning = 'none' could not align widths.

Changes

• Add PrepareOptions.fontKerning?: 'auto' | 'normal' | 'none' (default 'auto').
• Apply ctx.fontKerning in getFontMeasurementState on every prepare.
• Key the segment metric cache by font + kerning mode (default path keeps the plain font key).
• Document in README / CHANGELOG; unit tests with a fake canvas that models a kerning pair.

Test plan

• Unit: fontKerning: 'none' widens AV vs default; caches stay isolated.
• Package test suite / CI