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.
