ffi 1.10: multi-keyword Obj-C selectors through #objc_call

106/106 regression tests pass (+ffi-objc-call-08-multi-keyword).

`#objc_call(s32)(instance, "combine:and:", 7, 42)` round-trips
end-to-end via class_addMethod-registered IMP that does
`a * 100 + b` → 742. Pins three things:

1. The two-keyword selector "combine:and:" parses, mangles, and
   interns under the symbol `@OBJC_SELECTOR_REFERENCES_combine_and_`
   (every `:` → `_` — matches clang).
2. Multi-arg call lowering correctly puts arg0 / arg1 in the right
   slots after recv / sel.
3. The IMP-side sx fn signature `(self, _cmd, a: s32, b: s32)`
   with `callconv(.c)` interops with the Obj-C runtime's typical
   IMP shape, and the runtime forwards the keyword args to the
   right physical positions.

No codegen change — Phase 1.6's variadic-args branch in the
`objc_msg_send` lowering already handled this; this test just
locks in the surface.

examples/ffi-objc-call-08-multi-keyword.sx

@@ -0,0 +1,41 @@
+// Phase 1 step 1.10 (PLAN-FFI.md): multi-keyword Obj-C selectors
+// through `#objc_call`. Selector mangling matches clang's: every
+// `:` in the source-level selector becomes a `_` in the symbol
+// name of the cached SEL slot (so `initWithFrame:options:` →
+// `OBJC_SELECTOR_REFERENCES_initWithFrame_options_`).
+//
+// The selector is opaque text to the lowering — no codegen change
+// needed beyond Phase 1.6's variadic argument list. This test
+// pins that the round-trip works end-to-end via class_addMethod
+// + a real IMP that consumes both keyword args.
+
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+#import "modules/std/objc.sx";
+
+combine_imp :: (self: *void, _cmd: *void, a: s32, b: s32) -> s32 callconv(.c) {
+    a * 100 + b;
+}
+
+main :: () -> s32 {
+    inline if OS == .macos {
+        ns_object := objc_getClass("NSObject".ptr);
+        my_cls := objc_allocateClassPair(ns_object, "SxComboProbe".ptr, 0);
+
+        // Two-keyword selector: `combine:and:`.
+        // Method type encoding: i@:ii → returns int, implicit (self, _cmd),
+        // takes two ints. (`i` = int, `@` = id, `:` = SEL.)
+        sel := sel_registerName("combine:and:".ptr);
+        ok := class_addMethod(my_cls, sel, xx combine_imp, "i@:ii".ptr);
+        print("addMethod = {}\n", ok);
+        objc_registerClassPair(my_cls);
+
+        instance := class_createInstance(my_cls, 0);
+        r := #objc_call(s32)(instance, "combine:and:", 7, 42);
+        print("combine(7, 42) = {}\n", r);
+    }
+    inline if OS != .macos {
+        print("skipped (not macos)\n");
+    }
+    0;
+}