[ENG-46318] feat(webkit): add SplitButton (actions)

[HerbertJulio]

Summary

New SplitButton (actions) — a primary action button with an attached secondary trigger (dropdown/menu toggle), sharing the Button kind / size design axes.

Note: I did not author this component (it was an existing local branch); I verified it against our conventions before opening — spec .specs/split-button.md is status: approved with a matching checksum, no HEX/palette/raw-typography, no JS class presets, no <style>, TS-only, data-testid fallback actions-split-button, flat export ./split-button.

Notes

• Draft — pending review.
• The branch is ~8 commits behind dev; rebase before merge if package.json#exports conflicts.