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

Assembled from klammertext-dev f344a37d0f6e. 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/cond_test.sh

@@ -118,7 +118,7 @@ check_error() {
 FRAC='@@frac a | b : *a*/*b* @@'
 
 # A file for the @read non-strictness cases (21-21b).
-READ_FIXTURE=$(mktemp /tmp/cond_read.XXXXXX)
+READ_FIXTURE=$(mktemp "${TMPDIR:-/tmp}"/cond_read.XXXXXX)
 printf 'READ-FIXTURE\n' > "$READ_FIXTURE"
 trap 'rm -f "$READ_FIXTURE"' EXIT
 
@@ -248,7 +248,7 @@ check_eq "21b. ... so a missing file in a discarded branch is not an error" "ok"
 
 # An @eval side effect is the observable test: the branch either touched the
 # file or it did not.
-SIDE=$(mktemp -u /tmp/cond_side.XXXXXX)
+SIDE=$(mktemp -u "${TMPDIR:-/tmp}"/cond_side.XXXXXX)
 "$KTEXT" --klammersets none -s "@cond false | @eval :shell touch $SIDE @ | ok @" -d >/dev/null 2>&1
 if [ -f "$SIDE" ]; then
     echo "${red}FAIL${reset} 22. an @eval in a discarded branch ran"; FAIL=$((FAIL+1)); rm -f "$SIDE"