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/1231-ffi-extern-undeclared-lib.sx

@@ -0,0 +1,19 @@
+// An `extern LIB "csym"` reference must name something real, exactly like
+// its `#foreign LIB` twin (example 1620): `nosuchunit` names neither a
+// #library constant nor a named `#import c` unit, so this is a compile-time
+// diagnostic — the bogus library reference is caught BEFORE the symbol
+// would silently resolve through whatever image happens to carry it.
+//
+// Regression (FFI-linkage Part B): `checkForeignRefs` validated only a
+// `#foreign` (foreign_expr) library_ref and skipped the `extern` keyword's
+// `extern_lib`, so a bogus `extern` lib reference compiled silently (the
+// symbol resolved via the default image and ran). Prerequisite for
+// migrating the fn-decl `#foreign` path onto `extern`.
+#import "modules/std.sx";
+
+c_abs :: (n: i32) -> i32 extern nosuchunit "abs";
+
+main :: () -> i32 {
+    print("c_abs = {}\n", c_abs(-5));
+    0
+}

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";
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^