Add --tosvg switch to render seating as SVG, plus test images and docs

Sitzplatzverteilung.alsSVG() (libs/Strukturdaten.py) draws tables as
white circles and people as colored circles inside their table's
circle, colored by group membership. Table radius is derived from
the number of seats so all person-circles fit without touching; the
table-to-table grid spacing is derived from the largest table radius
so tables never overlap regardless of their Koordinaten values.

libs/platz.py gains a --tosvg switch that writes
<indir>/Sitzplatzverteilung.svg alongside the existing text/CSV
output.

The three unittest scenarios now each write their rendering to
doc/images/, and doc/docu_tests.md explains the three configurations
and what each test actually verifies (including why the "split
groups land on true neighbor tables" property only holds for a
specifically chosen seed in the circle scenario, not in general).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

doc/images/vier_tische_linie_viergruppen.svg

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" width="331.6" height="90.4" viewBox="0 0 331.6 90.4">
+<rect width="100%" height="100%" fill="white"/>
+<circle cx="45.2" cy="45.2" r="35.2" fill="white" stroke="black" stroke-width="1.5"/>
+<text x="45.2" y="8" text-anchor="middle" font-size="10" fill="black">Tisch 1</text>
+<circle cx="45.2" cy="32" r="12" fill="#e6194b" stroke="black" stroke-width="1"/>
+<circle cx="45.2" cy="58.4" r="12" fill="#e6194b" stroke="black" stroke-width="1"/>
+<circle cx="125.6" cy="45.2" r="35.2" fill="white" stroke="black" stroke-width="1.5"/>
+<text x="125.6" y="8" text-anchor="middle" font-size="10" fill="black">Tisch 2</text>
+<circle cx="125.6" cy="32" r="12" fill="#e6194b" stroke="black" stroke-width="1"/>
+<circle cx="125.6" cy="58.4" r="12" fill="#e6194b" stroke="black" stroke-width="1"/>
+<circle cx="206" cy="45.2" r="35.2" fill="white" stroke="black" stroke-width="1.5"/>
+<text x="206" y="8" text-anchor="middle" font-size="10" fill="black">Tisch 3</text>
+<circle cx="206" cy="32" r="12" fill="#3cb44b" stroke="black" stroke-width="1"/>
+<circle cx="206" cy="58.4" r="12" fill="#3cb44b" stroke="black" stroke-width="1"/>
+<circle cx="286.4" cy="45.2" r="35.2" fill="white" stroke="black" stroke-width="1.5"/>
+<text x="286.4" y="8" text-anchor="middle" font-size="10" fill="black">Tisch 4</text>
+<circle cx="286.4" cy="32" r="12" fill="#3cb44b" stroke="black" stroke-width="1"/>
+<circle cx="286.4" cy="58.4" r="12" fill="#3cb44b" stroke="black" stroke-width="1"/>
+</svg>