test(ffi-linkage): xfail example for extern data global (Phase 1.2c)

Add examples/1225-ffi-extern-global.sx — '__stdinp : *void extern;'
references libSystem's stdin pointer via the bare 'extern' modifier on
a typed var decl (the extern-named counterpart of the #foreign global
in examples/1205). Hand-authored snapshot expects the success output.

RED: 1225 is the sole corpus failure (636 ran, 1 failed) — parse error,
'extern' after a type annotation is not yet accepted in the var-decl
path. Phase 1.2d parses it and lowers the extern global.

xfail commit per the cadence rule.

current/CHECKPOINT-EXTERN-EXPORT.md

@@ -80,6 +80,9 @@ historical carve-out — keep `issues/*.md` provenance, gate the live tree only.
   extern_name → declare under C name, map sx→C) and extends the dedupe guard to
   extern. 1224 green (`c_abs`→`abs`); 1223 unregressed. Suite green (635/443).
   `green` commit. extern_lib parsed+stored (lib linking stays the `#library` axis).
+- (1.2c) Added `examples/1225-ffi-extern-global.sx` (`__stdinp : *void extern;`,
+  mirrors `#foreign` global 1205) + success snapshot. RED (636 ran, 1 failed — parse
+  error: var-decl `extern` not accepted). `xfail`; 1.2d greens it.
 
 ## Known issues
 - **Workflow hazard (1.2):** an editor format-on-save (or `zig fmt`) clobbered the