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.

mac/katomize.h

@@ -82,6 +82,10 @@ katom_list katomize_lines(const strings_t& lines, const std::string& source,
 // expand_compound_katom() and attributed to whichever rule the loop
 // happened to be holding, so it always read as "bar precedence".
 inline bool show_rewrite_rules = false;
+// Set it through this, never by assignment from a command: on Windows the
+// command's copy of an inline variable is not the library's (see
+// set_command_identity in command.h).
+void set_show_rewrite_rules(bool on);
 
 // Warn about text that matched no katom type and that no rule could divide
 // -- but only where it survives processing: text removed by #, ##, or