Two bugs in _emit_define_as_def: (1) nested def's _current_cell_vars
was replaced instead of unioned with parent — inner functions lost
access to parent's cell vars (skip_ws/skip_comment used bare pos
instead of _cells['pos']). (2) statement-context set! didn't check
_current_cell_vars, always emitting bare assignment instead of
_cells[...]. (3) nested functions that access parent _cells no longer
shadow it with their own empty _cells = {}.
Fixes UnboundLocalError in bootstrapped parser (sx_parse skip_ws)
that crashed production URL routing.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>