EvalContext.lookup now receives the full $var name, matching getCombined,
setBase, and getMods, and parseFactor throws "Unknown variable" when the
lookup returns undefined. /set and /roll use the new resolveVariable() so a
typo errors instead of silently evaluating to 0.
Declarations and tag modifiers stay lenient (unknown -> 0): the documented
$hp = $con*5+$mod_hp pattern depends on external/undeclared variables
defaulting to 0.