refactor(ffi-linkage): Phase 9.3 — rename *-foreign* example files → extern/runtime names

git-mv the 10 foreign-named example families to extern/runtime-class names + update
every #import/#include/#source ref, stale comment ref, and the 1172 stderr snapshot
(path + 'extern symbol' message). Renames: 0729…-foreign→…-extern, 1172-diagnostics-
foreign-symbol-conflict→…-extern-symbol-conflict, 1205/1207 ffi-foreign-global→
ffi-extern-global, 1216/1217 ffi-…-foreign-(in-method|result-chain)→…-extern-…,
1219-ffi-foreign→1219-ffi-extern, 1306 objc-foreign-class-chained→objc-runtime-class-
chained, 1318 objc-property-foreign→objc-property-extern-class. DEDUP: deleted
1218-ffi-foreign-cvariadic (identical to 1229-ffi-extern-cvariadic; updated 1229's
twin ref) + the orphaned 1620 dir. Also purged editors/vscode tmLanguage (#foreign
dropped from the directive highlighter) + 1220.h/issues-0030.sx comment refs. Suite
green (644 corpus / 443 unit, 0 failed).

examples/1306-ffi-objc-runtime-class-chained-dispatch.sx

@@ -0,0 +1,41 @@
+// Chained runtime-class method dispatch: `Cls.static().instance(...)`
+// resolves the inner call's return type so the outer dispatch's
+// receiver type is known. Pre-fix this collapsed to i64 in
+// `inferExprType`, the runtime_class_map lookup missed, and lowering
+// emitted `error: unresolved 'init'` (or 'initWithWindowScene' etc.)
+// — see issues/0043 for the chess uikit.sx C4 migration that hit it.
+//
+// Two return-type shapes covered: explicit `*ClassName` (alloc here)
+// and `*Self` (init). Both must propagate through the chain so the
+// next `.method(...)` finds the runtime-class declaration.
+
+#import "modules/std.sx";
+#import "modules/build.sx";
+
+NSObject :: #objc_class("NSObject") extern {
+    alloc :: () -> *NSObject;
+    init :: (self: *Self) -> *Self;
+}
+
+NSObjectSelfReturn :: #objc_class("NSObject") extern {
+    alloc :: () -> *Self;
+    init :: (self: *Self) -> *Self;
+}
+
+main :: () -> i32 {
+    inline if OS == .macos {
+        a := NSObject.alloc().init();
+        if a != null {
+            print("explicit-then-self ok\n");
+        }
+        b := NSObjectSelfReturn.alloc().init();
+        if b != null {
+            print("self-then-self ok\n");
+        }
+    }
+    inline if OS != .macos {
+        print("explicit-then-self ok\n");
+        print("self-then-self ok\n");
+    }
+    0
+}