Put the split button on its own geometry

It borrowed the connected group's corner variables, so its inner corners were
wrong at four of five sizes and shrank under the finger where SplitButton*Tokens
grows them; its chevron took the icon-button table's size at every step, sat dead
centre, and turned over on the expressive spring. groups.css now carries
--split-inner, --split-icon and --split-nudge per size, and the rotation uses
M3's standard scheme. Plan step 18, actions.md ACT-06, ACT-07, ACT-29, ACT-30.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

resources/views/components/split-button.blade.php

@@ -13,7 +13,11 @@
 
      Padding from SplitButton*Tokens (androidx Compose Material 3, Apache-2.0): the leading button
      keeps less room on its inner side at the two smallest sizes, and the trailing button is 48px
-     wide there. The corners are resources/css/components/groups.css. --}}
+     wide there. resources/css/components/groups.css carries the rest of the spec, which is all
+     geometry: the inner corners (4/4/4/8/12px, growing under the finger, not shrinking as a
+     connected group's do), the trailing icon's own size (22/22/26/38/50px, which the icon-button
+     table has no entry for), the 1/1/2/3/6px nudge that sits the chevron towards the leading half,
+     and the standard motion scheme M3 asks for while it turns over. --}}
 
 @props([
     'label' => null,