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/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