tkinter: load libtk at run time, so a Tk program starts without it

lib/pcl/tk.pas imports Tcl/Tk with external 'libtk8.6.so.0', so the binary names libtcl8.6 and libtk8.6 as required libraries and does not start at all on a box without them. A program that falls back to a text interface (Vloot: except ImportError / except tk.TclError) can only fall back when the library is present but no display is. Loading Tcl/Tk with dlopen at the first Tk() and raising TclError when it is missing would give one binary for both. The dlopen path exists (dynlibs, -dPXX_DYNLIB_LIBC) but is opt-in and verified on x86-64 only; aarch64 would need checking.

Seen in the field 2026-10-20 (neo-73): the Vloot binary built at add6990535 fails on via (real aarch64) even with --console, with "error while loading shared libraries: libtk8.6.so.0: cannot open shared object file". via has libtcl8.6 but not libtk8.6. A headless or lightly equipped fleet box therefore can't run the full-app binary in any mode. Raised to prio 30 because this is what makes one binary run everywhere.

Rejected 2026-10-20 (owner): auto-fallback to the console is a bad requirement. Build two applications, a console-only one (static) and a Tk one. Remote boxes only need the console, and only the workstation needs the GUI. A link-time libtk is how pxx is meant to work, so nothing changes here. The cheapest form of this ticket, if it ever comes back, is to replace the two libtk externals (Tk_Init, Tk_MainLoop) with package require Tk through Tcl_Eval, so that Tcl loads libtk itself. That needs no dlopen wrappers.