Hovering looks up the bounds of every selected object, and each lookup
went through the memo key check, which compares the whole selection. That
made a hover quadratic in the selection: 1.5 s at 50,000 selected paths.
The walk now checks the key once and shares it across its lookups.
Part of #1280.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>