Scope dimension selectors (x/y/z/t/offt/e/lon/lat/lev/time/ens/r) in
variable modifiers and function dimension args, e.g. ugrdprs(lev=700)
and ave(u,t=1,t=10). Selectors are anchored to a '(' (variable modifier,
excluding grouping parens) or ',' position and must be followed by
=value/+offset/-offset (gautil.c). Function calls consume their '(' so a
first-argument expression is not mistaken for a dimension. Adds syntax
tests covering the behavior and its documented edge cases.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>