test(ffi-linkage): xfail example for extern fn rename (Phase 1.2a)

Add examples/1224-ffi-extern-fn-rename.sx — 'c_abs :: (n) -> i32
extern "abs";' binds C's abs via the optional symbol-name override.
Hand-authored expected captures the success output (c_abs(-42) = 42).

RED: 1224 is the sole corpus failure (635 ran, 1 failed) — parse error,
the '"abs"' string after 'extern' is not yet accepted. Phase 1.2b
parses the optional [LIB] ["csym"] tail and consumes the rename.

xfail commit per the cadence rule.

examples/1224-ffi-extern-fn-rename.sx

@@ -0,0 +1,13 @@
+// extern with a "csym" rename (FFI-linkage stream, Phase 1.2): the sx name
+// `c_abs` binds C's `abs` via the optional symbol-name override after the
+// `extern` keyword — mirrors `#foreign "abs"`. The optional `LIB` ident slot
+// (extern_lib) sits before the string; here it's omitted (libc is
+// default-linked).
+#import "modules/std.sx";
+
+c_abs :: (n: i32) -> i32 extern "abs";
+
+main :: () -> i32 {
+    print("c_abs(-42) = {}\n", c_abs(xx -42));
+    0
+}

examples/expected/1224-ffi-extern-fn-rename.exit

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

examples/expected/1224-ffi-extern-fn-rename.stderr

@@ -0,0 +1 @@
+

examples/expected/1224-ffi-extern-fn-rename.stdout

@@ -0,0 +1 @@
+c_abs(-42) = 42