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.

kpy/klammertext.cpp

@@ -215,9 +215,12 @@ int instances = 0;
 // module, so a relative @read in read() text resolves against the directory
 // the script runs in -- the same rule ktext applies to -s input, and set at
 // each read() because a script may chdir between two of them.
+// The command's identity, through the library (set_command_identity): on
+// Windows an assignment to error.h's inline variables from this module
+// would land in this module's own copies, not the library's.
 void set_string_source()
 {
-    command_pathname = (fs::current_path() / "klammertext").string();
+    set_command_identity("klammertext", (fs::current_path() / "klammertext").generic_string());
 }
 
 PyObject* Machine_new(PyTypeObject* type, PyObject*, PyObject*)
@@ -607,7 +610,6 @@ PyMODINIT_FUNC PyInit_klammertext(void)
         return nullptr;
     }
     // What an error report names as the command.
-    command_name = "klammertext";
     set_string_source();
     const char* home = std::getenv(klammertext_home_var.c_str());
     PyObject* home_obj = home ? PyUnicode_FromString(home) : Py_None;