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.
f4b6cdae18