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

examples/1225-ffi-extern-global.sx

@@ -0,0 +1,15 @@
+// extern data global (FFI-linkage stream, Phase 1.2): reference a symbol
+// defined elsewhere (here libSystem's __stdinp) via the bare `extern`
+// linkage modifier on a typed var decl — the extern-named counterpart of
+// `<name> : <type> #foreign;` (see examples/1205). The optional
+// `extern [LIB] ["csym"]` tail mirrors the fn form; bare here (the sx name
+// IS the C symbol, resolved against the default-linked libSystem).
+#import "modules/std.sx";
+
+__stdinp : *void extern;
+
+main :: () -> i32 {
+    addr_bits : u64 = xx @__stdinp;
+    print("stdin extern global non-null: {}\n", addr_bits != 0);
+    0
+}

examples/expected/1225-ffi-extern-global.exit

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

examples/expected/1225-ffi-extern-global.stderr

@@ -0,0 +1 @@
+

examples/expected/1225-ffi-extern-global.stdout

@@ -0,0 +1 @@
+stdin extern global non-null: true