Fix BOM breaking bin/*.bat and add early Graphviz check to --tosvg wrappers

setenv.bat (and activate_venv/get_cmd/install_py.bat) had a UTF-8 BOM,
which cmd.exe read as part of the @echo off line, breaking every bin
script that calls setenv.bat via `call`. Stripped the BOM.

material_flow.bat/.sh and tro_flow.bat/.sh now check up front whether
the Graphviz engine --tosvg needs (dot, or neato with --use-cords) is
resolvable, and print an early WARNUNG with fix instructions if not -
before the CSV is even parsed. The Python scripts still run and report
the same failure in detail if the SVG step is actually reached.

doc/Python_Scripts.md

@@ -98,7 +98,11 @@ Conventions shared by the CLI tools:
   `ezdxf>=1.4.0` for `tro_annotate.py` only) — run `bin\install_py.bat` once.
 - `--tosvg` needs Graphviz's `dot` (or `neato`, for `tro_flow.py
   --use-cords`) on `PATH`, or the `GRAPHVIZ_DOT` environment variable pointing
-  at the `dot` executable.
+  at the `dot` executable. `bin/material_flow.bat`/`.sh` and `bin/tro_flow.bat`/`.sh`
+  check for the right engine up front when `--tosvg` is passed and print an early
+  `WARNUNG` if it's missing (`GRAPHVIZ_DOT` only covers `dot`, not `neato`) — this
+  is just a heads-up before the CSV is even parsed; the Python script still runs
+  and reports the same failure in detail (exit code `2`) if the SVG step is reached.
 
 ## 3. `material_flow.py` — material-flow graph of the mechanical layout