fix(fuselage): Stop clipping the SplitButton focus ring

[ggazzo]

Proposed changes (including videos or screenshots)

SplitButton rounded its corners with overflow: hidden on the group, which also clipped the focus ring each segment draws outside its box: a focused segment only showed a sliver of the ring over the adjacent segment (inherited from the old ButtonGroup joined variant).

The group no longer clips; the outer corners are rounded on the first and last segments instead (logical properties, so RTL keeps working). The resting look is unchanged — the existing SplitButton visual baselines pass as-is.

Trigger focused	Action focused
Full ring around the menu segment	Full ring around the action segment

Issue(s)

Seen while adopting SplitButton in the Rocket.Chat call controls (RocketChat/Rocket.Chat#42663).

Further comments

SplitButton has not been released yet (#2274), so this is a regression with no changeset.

[changeset-bot]

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