fix(ir): diagnose non-constant global initializers loudly (issue 0072)

globalInitValue's issue-0071 .identifier arm closed the bare-identifier hole,
but .field_access (and every other non-literal expression shape) still fell
through to `else => null`, so a global like `g : s32 = K.x;` was emitted with
no payload and silently zero-initialized (g=0).

Make the `else` emit a diagnostic — "global '<name>' must be initialized by a
compile-time constant" — instead of a null payload, so no unsupported shape can
silently zero. Two arms added alongside:

- `.null_literal => .null_val`: a `*void = null` global was previously a
  no-payload zero-init; this preserves the exact LLVMConstNull emission (fixes
  3 ffi examples that regressed on the first cut).
- explicit `.enum_literal => null` carve-out: the stdlib's
  `OS : OperatingSystem = .unknown;` zero-init is load-bearing for compile-time
  `inline if OS == .X`; documented, not folded into a silent fallthrough.

Field-access constant *evaluation* (materializing K.x -> 9) is intentionally
not implemented: a typed struct const like K is not registered in
module_const_map, so it would require new plumbing whose writes are read at
runtime — out of scope. The diagnostic is the issue-sanctioned outcome.

Regression: examples/1118-diagnostics-global-non-const-initializer-rejected.sx
(exit 1). Gate: zig build, zig build test, run_examples.sh -> 356/0.

examples/1118-diagnostics-global-non-const-initializer-rejected.sx

@@ -0,0 +1,19 @@
+// A top-level global initialized from a non-constant expression (here a field
+// access on a module constant, `K.x`) is rejected with a diagnostic. Without
+// the fix `registerTopLevelGlobal`'s init_val serializer handled only literals
+// / array / struct literals / identifiers and let every other shape fall through
+// to a null payload, so the global silently zero-initialized (`g=0`) — a wrong
+// value with no error.
+// Regression (issue 0072).
+// Expected: "global 'g' must be initialized by a compile-time constant"; exit 1.
+
+#import "modules/std.sx";
+
+Point :: struct { x: s32; y: s32; }
+K : Point : Point.{ x = 9, y = 4 };
+g : s32 = K.x;
+
+main :: () -> s32 {
+    print("g={}\n", g);
+    return g;
+}

examples/expected/1118-diagnostics-global-non-const-initializer-rejected.exit

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

examples/expected/1118-diagnostics-global-non-const-initializer-rejected.stderr

@@ -0,0 +1,5 @@
+error: global 'g' must be initialized by a compile-time constant
+  --> examples/1118-diagnostics-global-non-const-initializer-rejected.sx:14:11
+   |
+14 | g : s32 = K.x;
+   |           ^^^