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.

current/CHECKPOINT-EXTERN-EXPORT.md

@@ -71,6 +71,15 @@ historical carve-out — keep `issues/*.md` provenance, gate the live tree only.
   mirroring `foreign_expr`): `funcWantsImplicitCtx` + `declareFunction` cc +
   `lazyLowerFunction`/`lowerFunction`/`lowerFunctionBodyInto` guards. 1223 green;
   `declare i32 @abs(i32)` (C ABI, no ctx). Suite green (634/443). `green` commit.
+- (1.2a) Added `examples/1224-ffi-extern-fn-rename.sx` (`c_abs :: … extern "abs";`) +
+  hand-authored success snapshot (`c_abs(-42) = 42`). RED (635 ran, 1 failed — parse
+  error: `"abs"` after `extern` not yet accepted). `xfail`; 1.2b greens it. (Also
+  recovered a formatter-clobbered `parser.zig` — see Known issues.)
 
 ## Known issues
-None yet.
+- **Workflow hazard (1.2):** an editor format-on-save (or `zig fmt`) clobbered the
+  working-tree `src/parser.zig` between commits — it reformatted one-liners AND
+  silently dropped my `hasFnBodyAfterArrow` extern edit, reverting 1223 to a parse
+  error. Recovered with `git checkout src/parser.zig` (HEAD had the correct,
+  committed version). **After any Edit-tool change to a file the IDE may have open,
+  rebuild + run the affected example before trusting the edit.**

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