fix(0113): negated-literal global initializers fold as constants

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.

examples/0175-types-negative-literal-global.sx

@@ -0,0 +1,16 @@
+// A negated literal is a compile-time constant for a global initializer:
+// ints serialize directly, an integral negative float narrows into an
+// integer global (non-integral errors), and boundary values fit exactly.
+// Out-of-range negatives get the literal fits-check, not "non-constant".
+// Regression (issue 0113): `g : s64 = -1;` was rejected as not a
+// compile-time constant (globalInitValue had no unary_op arm).
+
+#import "modules/std.sx";
+
+g1 : s64 = -1;
+g2 : s64 = -4.0;
+g3 : s8 = -128;
+
+main :: () {
+    print("{} {} {}\n", g1, g2, g3);
+}

examples/expected/0175-types-negative-literal-global.exit

@@ -0,0 +1 @@
+0

examples/expected/0175-types-negative-literal-global.stderr

@@ -0,0 +1 @@
+

examples/expected/0175-types-negative-literal-global.stdout

@@ -0,0 +1 @@
+-1 -4 -128