test(ffi-linkage): Phase 5.0 prereq — xfail extern undeclared-library ref unvalidated

An `extern LIB "csym"` ref must name a declared #library / #import c unit,
like its `#foreign LIB` twin (example 1620). Today checkForeignRefs reads
only foreign_expr.library_ref and skips the extern keyword's extern_lib, so
a bogus `extern nosuchunit "abs"` compiles silently (the symbol resolves
via the default image and runs). Expected pins the DESIRED compile-time
diagnostic; the next commit extends checkForeignRefs to green it. Fourth
extern/#foreign divergence and a prerequisite for the fn-decl migration.

647 corpus (1231 xfail), 444 unit.

examples/expected/1231-ffi-extern-undeclared-lib.exit

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

examples/expected/1231-ffi-extern-undeclared-lib.stderr

@@ -0,0 +1,5 @@
+error: extern library 'nosuchunit' is not declared; expected a #library constant or a named '#import c' unit
+  --> examples/1231-ffi-extern-undeclared-lib.sx:14:1
+   |
+14 | c_abs :: (n: i32) -> i32 extern nosuchunit "abs";
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^