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/modulepath_test.sh

@@ -38,6 +38,10 @@ reset=$'\033[0m'
 # same directory and fail on the Mac only.  `pwd -P` is POSIX; macOS has no
 # GNU realpath by default.
 T="$(cd "$(mktemp -d)" && pwd -P)"
+# Under MSYS2 the physical path comes back in POSIX form (/tmp/...), which a
+# native ktext cannot open; the Windows spelling is what the fixtures and the
+# expectations must carry.
+command -v cygpath >/dev/null 2>&1 && T="$(cygpath -m "$T")"
 trap 'rm -rf "$T"' EXIT
 
 mkdir -p "$T/seta" "$T/setb" "$T/elsewhere"
@@ -51,6 +55,9 @@ check() {   # check NAME EXPECTED ARGS...
     local name="$1" expected="$2"; shift 2
     local out
     out=$(ktext "$@" --klammersets none -d 2>/dev/null)
+    # A working directory Python reports on Windows has backslashes; the
+    # expectation, built from the shell's Windows-form path, has slashes.
+    case "$(uname -s)" in MINGW*|MSYS*) out=${out//\\//};; esac
     if [ "$out" = "$expected" ]; then
         echo "${green}PASS${reset} $name"; PASS=$((PASS + 1))
     else