test(ffi-linkage): Phase 5.0 prereq — xfail extern same-name authors wrongly ambiguous

Two flat imports each declare `absval` via `extern libc "abs"` (the
`extern` twin of example 0729's `#foreign` form). Like its #foreign twin,
this must compile + run (prints 7), not error as an ambiguous bare-call
collision.

Today `isPlainFreeFn` / `isPlainFreeFnDecl` exclude a `#foreign` body but
classify an empty-block `extern` fn as a plain free function, so the two
extern authors ARE counted in the bare-call ambiguity verdict and the call
errors. A third extern/#foreign divergence (after visibility + variadic)
and a prerequisite for migrating the fn-decl `#foreign` path onto `extern`.

646 corpus (1230 xfail), 444 unit.

examples/1230-ffi-extern-same-name-authors/b.sx

@@ -0,0 +1,2 @@
+// The second flat author of `absval` — the identical `extern` binding.
+absval :: (n: i32) -> i32 extern libc "abs";