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.sx

@@ -0,0 +1,21 @@
+// Two flat FILE imports each declare the SAME libc symbol `absval` via the
+// `extern` keyword (the linkage-keyword twin of example 0729's `#foreign`
+// form). The bare-call resolver must NOT count extern authors when deciding
+// ambiguity — they are external C symbols, never rerouted by the bare-call
+// machinery, so the existing first-wins foreign/extern dispatch binds the
+// call and a same-name extern collision compiles + runs (prints 7), it does
+// NOT error as ambiguous.
+//
+// Regression (FFI-linkage Part B): `isPlainFreeFn` / `isPlainFreeFnDecl`
+// excluded a `#foreign` body but classified an empty-block `extern` fn as a
+// plain free function, so the two extern authors were wrongly counted as an
+// ambiguous bare-call collision. Prerequisite for migrating the fn-decl
+// `#foreign` path onto `extern`.
+#import "modules/std.sx";
+#import "1230-ffi-extern-same-name-authors/a.sx";
+#import "1230-ffi-extern-same-name-authors/b.sx";
+
+main :: () -> i32 {
+    print("absval = {}\n", absval(-7));
+    0
+}

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

@@ -0,0 +1,6 @@
+// One of two flat authors of `absval`, an `extern` libc binding — the
+// `extern` twin of example 0729's `#foreign libc "abs"`. A consumer
+// flat-importing BOTH must NOT see this as an ambiguous bare-call
+// collision: extern authors (external C symbols) are excluded from the
+// bare-call ambiguity verdict, exactly like their `#foreign` twins.
+absval :: (n: i32) -> i32 extern libc "abs";

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";

examples/expected/1230-ffi-extern-same-name-authors.exit

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

examples/expected/1230-ffi-extern-same-name-authors.stderr

@@ -0,0 +1 @@
+

examples/expected/1230-ffi-extern-same-name-authors.stdout

@@ -0,0 +1 @@
+absval = 7