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.

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