Person nodes rendered in `nodes` array order (backend/DB row order), so
Tab focus hopped between nodes unrelated to their on-screen position,
failing WCAG 2.4.3 Focus Order (Level A).
Render the node loop in reading order instead: sort by layout y (top
generation first) then x (left-to-right within a row), via a
`nodesInReadingOrder` derived. Nodes without a layout position sort last
(mirroring the `{#if pos}` guard); node.id is the final tie-break for a
total, deterministic comparator. Shift+Tab and reload-stability fall out
for free (reversed render order; x/y independent of backend order).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
