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.

issues/0072-global-field-access-const-initializer-silent-zero.md

@@ -0,0 +1,113 @@
+# 0072 — global field-access constant initializer silently zero-initializes
+
+> **RESOLVED.** Root cause: `Lowering.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 the
+> global was emitted with no payload and silently zero-initialized (`g=0`).
+> Fix: the `else` now emits a diagnostic — "global '<name>' must be initialized
+> by a compile-time constant" — instead of returning a null payload, so an
+> unsupported initializer shape can never silently zero. Two arms were added
+> alongside it: `.null_literal => .null_val` (a `*void = null` global was
+> previously a no-payload zero-init; this preserves that exact emission —
+> `LLVMConstNull`), and an 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) was
+> intentionally not implemented: a typed struct const like `K` is not registered
+> in `module_const_map`, so it would require new plumbing whose `module_const_map`
+> writes are read at runtime — out of scope; the diagnostic is the chosen,
+> issue-sanctioned outcome. Regression
+> `examples/1118-diagnostics-global-non-const-initializer-rejected.sx` (exit 1).
+
+## Symptom
+
+A top-level global initialized from a field access on a module constant compiles
+but is zero-initialized.
+
+Observed: `g=0`
+
+Expected: `g` should be initialized to `9`, or the compiler should reject the
+initializer loudly if field-access global constants are not supported yet.
+
+## Reproduction
+
+```sx
+#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;
+}
+```
+
+Run:
+
+```sh
+./zig-out/bin/sx run .sx-tmp/review-0071-field-access-const.sx
+```
+
+The repro is standalone; the inline source above is sufficient to recreate the
+scratch file under `.sx-tmp/`.
+
+## Investigation prompt
+
+Fix issue 0072: a top-level global initialized from a field access on a module
+constant must not silently become zero.
+
+Context:
+- This surfaced during Codex re-review of commit `ad7200c`, the issue-0071 fix.
+- `ad7200c` correctly handles identifier initializers that name module constants
+  (`K : A : 42; g : A = K;`) and diagnoses identifiers that are not usable
+  constants.
+- A remaining non-identifier expression shape still falls through silently:
+  `K : Point : Point.{ x = 9, y = 4 }; g : s32 = K.x;` emits a null global
+  initializer payload and runs as `g=0`.
+
+Suspected area:
+- `src/ir/lower.zig`, `Lowering.globalInitValue`.
+- The `.identifier` arm now diagnoses unusable identifier initializers, but the
+  generic `else => null` still covers `.field_access` and other expression
+  forms.
+- `constExprValue` currently handles literals, negative numeric literals, and
+  array literals; it does not evaluate field access or struct-literal module
+  constants. `constStructLiteral` can serialize direct struct literals when
+  called with the destination type, but that machinery is not used for `K.x`.
+
+Likely fix:
+- Add a loud diagnostic for unsupported top-level global initializer expression
+  shapes, or implement field-access constant evaluation in the same step.
+- If implementing it, resolve the base identifier through
+  `ProgramIndex.module_const_map`, materialize the base constant with its
+  recorded type, then extract the named field using the IR struct layout.
+- Do not replace the current bug with a real-type sentinel such as `.void`; an
+  unsupported initializer should either produce a concrete `ConstantValue` or a
+  user-visible diagnostic.
+- Preserve the issue-0071 behavior:
+  `K : A : 42; g : A = K;` must still emit `g=42`.
+- Preserve the issue-0070/0068 regressions:
+  `examples/0133-types-forward-alias-global.sx` and
+  `examples/1117-diagnostics-value-const-as-type-rejected.sx` must stay green.
+- Scope-check enum-literal globals separately. `OS : OperatingSystem = .unknown`
+  currently relies on compile-time constant injection and pre-existing zero-init
+  behavior; decide explicitly whether that stays carved out or gets its own
+  separate issue.
+
+Verification:
+- Repro above should either print `g=9` and exit `9`, or fail with a clear
+  "global initializer must be a compile-time constant" diagnostic. It must not
+  print `g=0`.
+- Run:
+
+```sh
+zig build
+zig build test
+bash tests/run_examples.sh
+```