test(ffi-linkage): Phase 8.0 — xfail #foreign-removed rejection diagnostic

Add examples/1176-diagnostics-foreign-removed.sx pinning the DESIRED Phase 8 cutover
behavior: a bare '#foreign' decl must be rejected with a clear migration message
('#foreign has been removed; use the postfix extern/export'). RED — '#foreign' still
parses (routes onto extern) so the decl compiles and exits 0 instead of erroring.
The very next commit (8.1, parser hard-reject) greens it.

examples/1176-diagnostics-foreign-removed.sx

@@ -0,0 +1,10 @@
+// Phase 8 (FFI-linkage) cutover: the prefix `#foreign` linkage directive has
+// been removed. A declaration imports an external C symbol via the postfix
+// `extern` keyword (or defines + exposes one via `export`). The parser rejects
+// `#foreign` with a clear migration message instead of routing it onto `extern`.
+//
+// Expected: one error on the `#foreign` token; exit 1.
+
+abs_c :: (n: i32) -> i32 #foreign;
+
+main :: () -> i32 { 0 }

examples/expected/1176-diagnostics-foreign-removed.exit

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

examples/expected/1176-diagnostics-foreign-removed.stderr

@@ -0,0 +1,5 @@
+error: `#foreign` has been removed; use the postfix `extern` (import) / `export` (define) linkage keyword instead
+  --> examples/1176-diagnostics-foreign-removed.sx:8:26
+   |
+ 8 | abs_c :: (n: i32) -> i32 #foreign;
+   |                          ^^^^^^^^