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/command_option_test.sh

@@ -136,7 +136,7 @@ echo "-- the output policy: three categories, two streams --"
 # For ktext, category 3 is a DOCUMENT that may be piped, so nothing else may
 # share the stream.  It did: K::log and msg() both wrote to stdout, so
 # "ktext -d -v 1 > doc.txt" put the whole argument dump inside the document.
-OUTF=$(mktemp /tmp/kout.XXXXXX); ERRF=$(mktemp /tmp/kerr.XXXXXX)
+OUTF=$(mktemp "${TMPDIR:-/tmp}"/kout.XXXXXX); ERRF=$(mktemp "${TMPDIR:-/tmp}"/kerr.XXXXXX)
 trap 'rm -f "$OUTF" "$ERRF"' EXIT
 
 ktext --klammersets none -s 'hello' -d -v 1 > "$OUTF" 2> "$ERRF"