testplan: simple hard drops
implemented hard drops
cleaned up stamp-shape by reusing (current-blocks)
learntris: picolisp coding standards for (xyvs-clip)
learntris.l: extracted generalized (xyvs-join) from (xyvs-update) to prepare for collision detection.
learntris.l: fixed (nudge-v) to always return T/NIL and corrected (hard-drop) logic
learntris.l: generalized collision detection for nudge-v
added ?b to show current bounds
testplan: simplified collision tests