fix(ir): resolve forward alias in top-level global annotations (issue 0070)

Issue 0069's resolveForwardIdentifierAliases fixpoint runs at the END of
scanDecls, but top-level var_decl globals and typed module constants had
their annotations resolved via resolveType(ta) inside the SAME scan loop,
before the fixpoint. So a forward identifier alias (`A :: B; B :: s32;`)
used as a global's type (`g : A = 7;`) was still absent from
type_alias_map: resolveType fabricated an empty-struct stub, and the global
got a type mismatching its initializer at LLVM verification (the typed-const
path `K : A : 42;` silently mistyped the constant instead).

Split scanDecls into two passes: pass 1 registers function/type/alias facts,
then resolveForwardIdentifierAliases converges the aliases, then pass 2
registers var_decl globals (registerTopLevelGlobal) and typed module
constants (registerTypedModuleConst) against the converged alias map.
Globals/typed-consts can't be named in a type position, so deferring them
past type/alias registration is order-safe; the untyped module-const branch
(no annotation to resolve) stays in pass 1.

One incidental IR snapshot reorder (examples/1309: user globals now emit
after foreign-class globals — semantically identical, program still exits 0).

Regression: examples/0133-types-forward-alias-global.sx (forward-alias global
+ typed const). Gate: zig build, zig build test, run_examples.sh -> 354/0.

issues/0070-forward-alias-global-annotation-before-fixpoint.md

@@ -0,0 +1,113 @@
+# 0070 — forward alias in top-level global annotation reaches LLVM verifier
+
+> **RESOLVED.** Root cause: issue 0069's `resolveForwardIdentifierAliases`
+> fixpoint runs at the END of `Lowering.scanDecls`, but the same scan loop
+> resolved top-level `var_decl` global annotations (and typed module-constant
+> annotations) via `self.resolveType(ta)` BEFORE that fixpoint ran — so a forward
+> alias (`A :: B; B :: s32; g : A = 7;`) was still absent from
+> `type_alias_map`, `resolveType` fabricated an empty-struct stub, and the global
+> got a type mismatching its initializer at LLVM verification (the typed-const
+> path silently mistyped the constant instead).
+> Fix: split `scanDecls` into two passes. Pass 1 registers function/type/alias
+> facts; then `resolveForwardIdentifierAliases` converges the aliases; then pass 2
+> registers top-level `var_decl` globals (`registerTopLevelGlobal`) and typed
+> module constants (`registerTypedModuleConst`), so their annotations resolve
+> against the converged alias map. Globals/typed-consts can't be named in a type
+> position, so deferring them past type/alias registration is order-safe; the
+> untyped module-const branch (no annotation to resolve) stays in pass 1.
+> Regression: `examples/0133-types-forward-alias-global.sx`.
+
+## Symptom
+
+A forward identifier type alias used as a top-level global's type annotation
+does not resolve before the global is registered, producing an LLVM verifier
+failure instead of compiling as the alias target type.
+
+Observed:
+
+```text
+LLVM verification failed: Global variable initializer type does not match global variable type!
+ptr @g
+```
+
+Expected: `A :: B; B :: s32; g : A = 7;` should type `g` as `s32` and compile/run
+the same way as the ordered alias form.
+
+## Reproduction
+
+```sx
+A :: B;
+B :: s32;
+
+g : A = 7;
+
+main :: () -> s32 {
+    return g;
+}
+```
+
+Run:
+
+```sh
+./zig-out/bin/sx run .sx-tmp/probe-0069-forward-alias-global.sx
+```
+
+The repro is standalone; the inline source above is sufficient to recreate the
+scratch file under `.sx-tmp/`.
+
+## Investigation prompt
+
+Fix issue 0070: a forward identifier type alias used in a top-level global
+annotation must resolve before that global's type is registered.
+
+Context:
+- Issue 0069 (`49a383d`) added `Lowering.resolveForwardIdentifierAliases`, a
+  fixpoint post-pass at the end of `scanDecls`, to resolve top-level
+  identifier-RHS aliases like `A :: B; B :: s32;`.
+- That works for aliases used later in function bodies because the A2.4
+  unknown-type pass and body lowering run after `scanDecls`.
+- But top-level `var_decl` annotations are resolved inside the same `scanDecls`
+  loop before `resolveForwardIdentifierAliases(decls)` is called. So
+  `g : A = 7;` can be typed while `A` is still absent from
+  `ProgramIndex.type_alias_map`.
+- Suspected area: `src/ir/lower.zig`, `Lowering.scanDecls`, especially the
+  ordering between `.const_decl` alias collection, the new
+  `resolveForwardIdentifierAliases`, and the `.var_decl` branch that calls
+  `self.resolveType(ta)`.
+
+Likely fix:
+- Split the scan ordering so all top-level type declarations and identifier
+  aliases converge before any top-level global annotation is resolved.
+- One possible shape: first scan/register function/type/alias facts, run the
+  forward-alias fixpoint, then handle top-level `var_decl` global registration
+  and literal module constants that require resolved annotation types.
+- Do not reintroduce issue 0068: `NotAType :: 123; v: NotAType` must still emit
+  `unknown type 'NotAType'`.
+- Do not fabricate stubs while trying to resolve the forward alias. The alias
+  facts should still come from `ProgramIndex.type_alias_map` and real
+  `TypeTable.findByName` hits.
+
+Verification:
+- Add a focused regression, likely in the `01xx` types block:
+
+```sx
+A :: B;
+B :: s32;
+g : A = 7;
+main :: () -> s32 { return g; }
+```
+
+- Keep `examples/0132-types-forward-type-alias.sx`,
+  `examples/0116-types-type-alias-size-align.sx`,
+  `examples/0201-generics-generic-struct.sx`, and
+  `examples/1117-diagnostics-value-const-as-type-rejected.sx` green.
+- Run:
+
+```sh
+zig build
+zig build test
+bash tests/run_examples.sh
+```
+
+Expected result: the forward-alias global program exits 7, issue 0068 remains
+rejected with a diagnostic, and the full suite passes.