FX.155 CadCraft: no Arc drop-down in the default toolbar, and the ribbon Arc menu showed 6 of its 12 tools

CadCraft default toolbar (ui.inc): Arc and Circle had no drop-down; the eleven arc methods and the two circle methods sat only under Draw > More. Each now has a caret beside the button listing its ways to draw (moved out of More). FX.137 added the eleven ways to draw an arc, but the shared menu code (assets/js/app-input.js) kept only the first 6 tools of any split button, and dropped the rest without a word. CadCraft's Arc and PartCraft's sketch Arc (12 tools each) lost half their list; no other built-in split in any app is longer than 6. The limit is now 16, and a split cut to fit is reported like any other bad menu entry instead of vanishing. New test tools/shell-test/input-splits-test.mjs runs every built-in menu style of every app through the sanitiser and fails if anything is dropped (it fails on the old limit of 6, passes on 16). Installed 0.9.0 desktop builds carry their own copy of the menu code, so they need the 0.9.1 build (docs/0.9.1.md, 91.1); the web edition has the fix now.