Improve contextual fraction shaping

[devs6186]

Summary

• Reworks the proportional frac feature so whole numbers stay as default figures.
• Keeps existing standalone precomposed fractions and uses contextual numerator/denominator shaping for multi-digit fractions.
• Regenerates proportional OTF, TTF, variable, and WOFF2 outputs.

Verification

• gftools builder sources/config.yaml
• Focused HarfBuzz shaping check for 123, 1/2, 12/34, and 123/456 on representative proportional TTF, OTF, and variable fonts.

FontBakery note: the current upstream baseline is already red under the available WSL FontBakery environment. A local upstream-vs-change comparison during prep found no new FAIL or ERROR results introduced by the prepared changes.

Closes #88

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