test+fix(stammbaum): enlarge marriage-line midpoint dot to r=6 (#361)

Once the dot starts stacking to disambiguate multiple marriages on
multi-spouse rows it carries meaning, so it's no longer decorative —
WCAG 1.4.11 (3:1) applies. r=6 (12 px diameter) covers the contrast
gap; the existing brand-navy fill against the gutter and surface
backgrounds satisfies the ratio without a hue change.

Impl-ref table in stammbaum-tree-spec.html updated to match (r=6 /
12 px dia / Informational), with the WCAG reference noted.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/person/genealogy/StammbaumTree.svelte

@@ -319,7 +319,7 @@ const parentLinks = $derived.by<ParentLinks>(() => {
 			<circle
 				cx={(aCenter.x + bCenter.x) / 2}
 				cy={(aCenter.y + bCenter.y) / 2}
-				r="4.5"
+				r="6"
 				fill="var(--c-primary)"
 			/>
 		{/if}