Klammertext runs natively on Windows; a Windows source-install guide; the bundled sh for @eval :shell

Assembled from klammertext-dev f344a37d772c. The commands, the library,
the Standard Klammer Set, the reflection klammerset and the Python
binding build and run natively on Windows (MSYS2 UCRT64 gcc, python.org
Python), and both test tiers pass there. doc/install/windows_source_install.md
is the fifth install guide: a source install parallel to the Linux and
macOS ones, verified by following it on a pristine Windows 11.

Behaviour on every system: paths Klammertext prints use forward slashes;
output is LF; the command line is UTF-8 on Windows; @eval :shell is a
POSIX shell command everywhere -- on Windows it runs the sh Klammertext
ships, BusyBox for Windows (win/busybox/, GPLv2, with its licence and the
source pointer). KLAMMERTEXT_FONTS and KLAMMERTEXT_KLAMMERSETS are lists in
the system's PATH convention (';' on Windows). On Linux and macOS nothing
observable changes.

tst/editor_test.sh

@@ -91,17 +91,20 @@ check() {   # check NAME FILE_A FILE_B
 }
 
 # --- the shared core: API and CLI (required) -------------------------------
-if ! command -v python3 >/dev/null 2>&1; then
-    echo "editor_test.sh: python3 not found" >&2
+# The interpreter's name: python3 on Linux and macOS; on Windows python.org
+# installs "python" only, and env/runtime.env exports PYTHON there.
+PYTHON=${PYTHON:-python3}
+if ! command -v "$PYTHON" >/dev/null 2>&1; then
+    echo "editor_test.sh: $PYTHON not found" >&2
     exit 1
 fi
-if ! PYTHONDONTWRITEBYTECODE=1 python3 "$FIX/editor_driver.py" "$SHARED_DIR" "${py_args[@]}"; then
+if ! PYTHONDONTWRITEBYTECODE=1 "$PYTHON" "$FIX/editor_driver.py" "$SHARED_DIR" "${py_args[@]}"; then
     echo "editor_test.sh: the Python driver failed" >&2
     exit 1
 fi
 
 # --- the Sublime adapters resolve to the shared core -----------------------
-if PYTHONDONTWRITEBYTECODE=1 python3 -c "
+if PYTHONDONTWRITEBYTECODE=1 "$PYTHON" -c "
 import sys
 sys.path.insert(0, '$SUBLIME_DIR')
 import Klammertext, Klammertext_indent, Klammertext_align
@@ -116,7 +119,7 @@ else
 fi
 
 # --- the language server protocol test -------------------------------------
-if PYTHONDONTWRITEBYTECODE=1 python3 "$FIX/ls_test.py" "$SHARED_DIR" "$FIX" >"$OUT/ls.log" 2>&1; then
+if PYTHONDONTWRITEBYTECODE=1 "$PYTHON" "$FIX/ls_test.py" "$SHARED_DIR" "$FIX" >"$OUT/ls.log" 2>&1; then
     echo "${green}PASS${reset} language server protocol ($(grep -c '^PASS' "$OUT/ls.log") checks)"
     PASS=$((PASS + 1))
 else
@@ -255,7 +258,7 @@ fi
 mkdir -p "$OUT/vsix"
 cp -R "$EDIT_DIR/vscode" "$EDIT_DIR/shared" "$OUT/vsix/"
 if ( cd "$OUT/vsix/vscode" && ./make_vsix.sh >/dev/null 2>&1 ) && \
-   PYTHONDONTWRITEBYTECODE=1 python3 -c "
+   PYTHONDONTWRITEBYTECODE=1 "$PYTHON" -c "
 import sys, zipfile
 names = set(zipfile.ZipFile('$OUT/vsix/vscode/klammertext.vsix').namelist())
 need = {'extension.vsixmanifest', '[Content_Types].xml',