feat(ffi-linkage): reject extern+export on one decl (Phase 4) — 1175 green

src/parser.zig

@@ -2002,6 +2002,14 @@ pub const Parser = struct {
         // Optional postfix linkage modifier: `extern` (import) / `export` (define).
         const extern_export = self.parseOptionalExternExport();
 
+        // `extern` and `export` are mutually exclusive — one declaration is either
+        // an import or a definition, never both. Reject the redundant second keyword
+        // with a clear message rather than the bare "expected ';'" the body parser
+        // would otherwise emit.
+        if (extern_export != .none and (self.current.tag == .kw_extern or self.current.tag == .kw_export)) {
+            return self.fail("conflicting linkage: 'extern' and 'export' cannot be combined — a declaration is either an import ('extern') or a definition ('export')");
+        }
+
         // Optional `[LIB] ["csym"]` tail after extern/export — a library-alias
         // ident then a C symbol-name string, both optional (mirrors
         // `#foreign LIB "csym"`). Stored on extern_lib/extern_name; the rename