globalInitValue had no unary_op arm, so g : s64 = -1; fell into the
catch-all 'must be initialized by a compile-time constant' even though
constExprValue already folds negate(literal) for the module-const
identifier route. The new arm routes through constExprValue and applies
the direct-literal rules to the folded value: checkIntLiteralFits on
ints (g : s8 = -300 gets the range diagnostic), and a negated float at
an integer global narrows only when integral (-4.0 folds to -4, -4.5
errors). Binary-op initializers keep the specific non-constant
diagnostic.
Regression: examples/0175-types-negative-literal-global.sx.
