test(ffi-linkage): Phase 5.0 prereq — xfail lib-less extern/#foreign C-import visibility equivalence

Cross-module example (main → b → c) referencing c's lib-less C imports
transitively. The non-transitive C-import gate (lower/decl.zig
c_import_bare) must police the legacy `#foreign` form and the new
`extern` keyword IDENTICALLY — same 'C function not visible' diagnostic,
not the generic top-level-name wording. Today the extern twin escapes the
c_import_bare gate (body is an empty block, not foreign_expr) and is only
caught by the general isNameVisible gate, yielding the generic message.
Expected snapshot pins the DESIRED equivalent wording; the next commit
aligns the gate to green it. Prerequisite for migrating the fn-decl
`#foreign` path onto `extern`.

443/444 corpus (1228 xfail), 444 unit.

examples/expected/1228-ffi-extern-c-non-transitive.exit

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

examples/expected/1228-ffi-extern-c-non-transitive.stderr

@@ -0,0 +1,11 @@
+error: C function 'c_foreign_abs' not visible; add #import for the module that declares it
+  --> examples/1228-ffi-extern-c-non-transitive.sx:16:19
+   |
+16 |     print("{}\n", c_foreign_abs(-3));
+   |                   ^^^^^^^^^^^^^
+
+error: C function 'c_extern_abs' not visible; add #import for the module that declares it
+  --> examples/1228-ffi-extern-c-non-transitive.sx:17:19
+   |
+17 |     print("{}\n", c_extern_abs(-4));
+   |                   ^^^^^^^^^^^^