ffi 0.10: extend 94-foreign-global with cross-file companion

97/97 regression tests pass (94 expected updated; +issue-0037 from
the prior commit).

The companion `94-foreign-global-helper.sx` ALSO declares
`__stdinp : *void #foreign;`. Two sx files referencing the same
extern symbol must link cleanly — LLVM dedupes the named global at
the module level, and the C linker resolves both refs to the one
libSystem definition.

The full ergonomic story (helper computes the *same* address as the
main file's direct read) is blocked on issue-0037: lower.zig's
`address_of(global)` branch produces `undef` when the body is a
non-main function, even single-file. Once issue-0037 closes, fold
the helper's address back into an equality check here.

The cross-file link itself works today and is the lemma we're
locking in. This is also the closest thing today to the cross-file
extern-global ergonomic issue-0030 wants — `#foreign` already works
across files; the missing piece is sx-side `extern` decls for
sx-defined globals.

examples/94-foreign-global-helper.sx

@@ -0,0 +1,13 @@
+// Companion module for examples/94-foreign-global.sx (PLAN-FFI 0.10).
+// Declares the same `#foreign` extern global as the main file; the
+// linker should treat both decls as one symbol. We deliberately don't
+// READ `@__stdinp` from inside a helper fn body — that path is busted
+// today (see examples/issue-0037.sx) — we just expose a trivial fn so
+// this file participates in the link and the cross-file decl
+// coexistence is exercised.
+
+__stdinp : *void #foreign;
+
+stdinp_addr_present :: () -> s32 {
+    1;
+}

examples/94-foreign-global.sx

@@ -2,13 +2,31 @@
 // reference libSystem / framework symbols (NSConcreteStackBlock,
 // __stdinp, etc.) for FFI bridges. Mirrors the long-standing
 // `<fn> :: (...) -> ... #foreign;` form on the function side.
+//
+// Cross-file dimension (PLAN-FFI step 0.10): the helper companion
+// `94-foreign-global-helper.sx` ALSO declares `__stdinp : *void #foreign;`.
+// Both files referencing the same extern symbol must link cleanly —
+// LLVM dedupes the named global, the C linker resolves both refs to
+// the one libSystem symbol.
+//
+// We *don't* check that the helper computes the same address — see
+// issue-0037 (helper-function-scoped `@foreign_global` lowers to
+// undef today). When that fixes, fold the helper's address back into
+// the equality check here.
 
 #import "modules/std.sx";
+#import "94-foreign-global-helper.sx";
 
 __stdinp : *void #foreign;
 
 main :: () -> s32 {
   addr_bits : u64 = xx @__stdinp;
   print("stdin extern global non-null: {}\n", addr_bits != 0);
+  // Force the helper symbol to participate in linking (otherwise the
+  // imported file's #foreign decl might get dropped by the
+  // dead-code stripper). The actual return value is busted today
+  // — see issue-0037.
+  _ := stdinp_addr_present();
+  print("helper file linked: true\n");
   0;
 }

tests/expected/94-foreign-global.txt

@@ -1 +1,2 @@
 stdin extern global non-null: true
+helper file linked: true