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>

README.md

@@ -121,6 +121,10 @@ A run writes two files into the `--indir` directory:
 - `TischePersonen.txt` — table → seated people, with venue/table/seat numbers.
 - `PersonenTische.csv` — person → table, sorted alphabetically by name.
 
+Pass `--tosvg` to additionally write `Sitzplatzverteilung.svg` into the `--indir`
+directory: tables are drawn as white circles, people as colored circles inside their
+table's circle (color = group). See `doc/docu_tests.md` for example renderings.
+
 ## Further details
 
 See `CLAUDE.md` for a deeper architectural walkthrough of the GA framework and domain