tk.X / tk.Y are unreachable because tkinter.pas spells them X_ / Y_

Found building songformatter (2026-10-27, peer session): fill=tk.X reported "no member X came of the qualifier tk". Pascal folds case, so tkinter.pas declares X_ and Y_. PyMapReservedMember is a fixed list of reserved words, and adding X there would rename m.x on every user module.

Fix. The NilPy qualified-member arm asks the unit instead. When it declares nothing called name but does declare name_ as a constant, the underscored spelling is taken. The rule is plain-first, as the reserved-word arm beside it is. A one-character 'x' is a char constant and lives in the symbol table, so both tables are asked.

Test: examples/tk/tk_x_and_y_constants.npy (compiled, not run; checked under Xvfb by hand: prints x y both).