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/1228-ffi-extern-c-non-transitive.sx

@@ -0,0 +1,19 @@
+// `#import` is non-transitive for C-import functions: main imports b,
+// b imports c, so main must NOT see c's lib-less `#foreign` / `extern`
+// C functions directly. Referencing either is rejected by the
+// C-import visibility gate (lower/decl.zig `c_import_bare`).
+//
+// Equivalence lock (FFI-linkage Part B): the legacy `#foreign` form and
+// the new `extern` keyword are two spellings of the same lib-less
+// C-symbol import, so they must produce the SAME "C function not
+// visible" diagnostic — not the generic top-level-name wording. This
+// pins that equivalence as a prerequisite for migrating the fn-decl
+// `#foreign` path onto `extern`.
+#import "modules/std.sx";
+#import "1228-ffi-extern-c-non-transitive/b.sx";
+
+main :: () -> i32 {
+    print("{}\n", c_foreign_abs(-3));
+    print("{}\n", c_extern_abs(-4));
+    return 0;
+}

examples/1228-ffi-extern-c-non-transitive/b.sx

@@ -0,0 +1,7 @@
+// Intermediate module: directly imports c.sx, so BOTH of c's lib-less
+// C functions are legitimately visible here (the legal usage site).
+#import "c.sx";
+
+b_use :: () -> i32 {
+    return c_foreign_abs(-1) + c_extern_abs(-2);
+}

examples/1228-ffi-extern-c-non-transitive/c.sx

@@ -0,0 +1,7 @@
+// Two spellings of the same lib-less C-symbol import: the legacy
+// `#foreign` form and the new `extern` keyword. Both declare a C
+// function resolved at link time with no library reference. The
+// FFI-linkage migration (Part B) requires the two to be policed
+// IDENTICALLY by the non-transitive C-import visibility gate.
+c_foreign_abs :: (x: i32) -> i32 #foreign;
+c_extern_abs  :: (x: i32) -> i32 extern;

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));
+   |                   ^^^^^^^^^^^^