Move the layout function out of StammbaumTree.svelte (lines 47-275) into a
new pure TypeScript module at frontend/src/lib/person/genealogy/layout/
buildLayout.ts so it can be exercised by direct unit tests. Drops the
eslint-disable svelte/prefer-svelte-reactivity blanket; switches the
remaining scope-local Maps/Sets in parentLinks to SvelteMap/SvelteSet to
satisfy the rule per-call-site. No behaviour change — existing
StammbaumTree tests must pass byte-for-byte.
Refs #689
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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