Windows backslashes in TCL strings are interpreted as escape sequences
(\f → form feed, \t → tab). This caused 'dow "Files\fsbl..."' to
fail with garbled paths — TCL couldn't find the file.
Added _tcl_path() helper: replaces \ with / in any path before
embedding in TCL. Forward slashes work on all platforms including
Windows. Applied to all 7 path sites: source, fpga -file, dow.