Group parallel Gefaellestrecken in the material-flow diagram

A bank of gravity lanes between the same two Kreisel (same slope) was
drawn as one box per lane, making large storage banks unreadable.
render_dot now collapses such lanes into a single Gefaellestrecke-styled
block with a double border, labelled with the id range and combined
Separator/Scanner ranges. Purely visual - the graph, --doc report and
--connect output still list every lane individually.

doc/Python_Scripts.md

@@ -132,6 +132,16 @@ Reads the CSV export and builds a directed graph of the mechanical objects:
 > will contain the 61 conveyor items but **no** BTMT rows, so no entry/exit nodes
 > appear. The recognition above is ready for when BTMT rows are present.
 
+In the rendered graph, **parallel Gefällestrecken between the same two Kreisel
+lanes with the same slope** (`Hoehe_Von_mm`/`Hoehe_Bis_mm`) — a typical
+line-storage bank — are drawn as a **single** Gefällestrecke-styled box with a
+double border (`peripheries=2`) instead of one box per line. Its label states
+the id range and count of the merged lines plus the combined Separator/Scanner
+id ranges, e.g. `Gefaellestrecke 0017 - 0036 (20x)` / `Separator (20): 0037 -
+0056`. This is purely a rendering simplification (`_group_parallel_gefaelle`
+in `lib/material_flow.py`) — the underlying graph, `--doc` report and
+`--connect` output are unaffected and still list every line individually.
+
 Flow direction between conveyor segments is derived from height change
 (`Hoehe_Von_mm`/`Hoehe_Bis_mm`) or, failing that, from `Antriebfahrtrichtung`
 ("Auf"/"Ab"); connections whose direction can't be determined are drawn