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.
