fix(ir): materialize global initialized from module const (issue 0071)

registerTopLevelGlobal's init_val switch serialized only literal / array-
literal / struct-literal initializers. An identifier initializer
(`K : A : 42; g : A = K;`) fell through to `else => null`, so the global was
emitted with no payload and silently zero-initialized (printed g=0).

Extract the initializer serialization into globalInitValue and add an
.identifier arm that materializes the global's static value from
ProgramIndex.module_const_map (typed module consts are registered in the same
scanDecls pass-2 just before, via registerTypedModuleConst). An identifier
that names no usable constant now emits a diagnostic instead of silently
zeroing — a global has no run site for a dynamic initializer.

Other initializer shapes (enum-literal shorthand, etc.) keep their established
static-lowering behavior; enum-literal globals' zero-init is load-bearing for
`inline if OS == ...` in the stdlib, so it stays out of scope here. This pass
only closes the identifier/module-const hole.

Regression: examples/0134-types-global-init-from-module-const.sx (g=42, exit
42). Gate: zig build, zig build test, run_examples.sh -> 355/0.

examples/0134-types-global-init-from-module-const.sx

@@ -0,0 +1,18 @@
+// Top-level global initialized from a module constant copies the constant's
+// value (not a silent zero). `K : A : 42; g : A = K;` resolves the forward
+// alias `A` to `s32` and materializes `g`'s static initializer from `K`.
+// Regression (issue 0071): `registerTopLevelGlobal`'s init_val switch only
+// handled literals/array/struct literals; an identifier initializer fell
+// through to a null payload and the global silently zero-initialized.
+#import "modules/std.sx";
+
+A :: B;
+B :: s32;
+
+K : A : 42;
+g : A = K;
+
+main :: () -> s32 {
+    print("g={}\n", g);
+    return g;
+}

examples/expected/0134-types-global-init-from-module-const.exit

@@ -0,0 +1 @@
+42

examples/expected/0134-types-global-init-from-module-const.stdout

@@ -0,0 +1 @@
+g=42