A placed ability's position is the top-left of its box, and a circle is
centered in that box. #258 shrank Steel Garden's radius and with it the
box, so every saved Steel Garden drew about 2.25 m up and left.

CircleAbility now takes a storedSize: the size saved positions were placed
at. The box keeps that size and the circle draws at the new radius in its
middle, so saved positions keep their meaning in local libraries, .ica
files and the cloud, with no migration. Agents already do the same for
their marker size (storedAgentAnchor).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>