ffi 1.3 make-green: #objc_call(void)(recv, "sel:") codegen

100/100 regression tests pass; ffi-objc-call-02-void-return flips
from xfail (codegen rejection) to passing ("ok").

Lowering for `#objc_call(void)(recv, "selector:")` lands in
lower.zig as `lowerFfiIntrinsicCall`:

  %sel  = call ptr @sel_registerName(<"selector:">)
  %call = call ptr @objc_msgSend(<recv>, %sel)

Two extern decls (`sel_registerName(*u8) -> *void` and
`objc_msgSend(*void, *void) -> *void`) are declared lazily and
cached on the Lowering instance via `objc_msg_send_fid` /
`sel_register_name_fid`, so multiple call sites share one
declaration each.

Phase 1.3 deliberately keeps scope tight: only `void` return + just
(recv, selector) arity is wired. Non-void returns + variadic arity
fall through with a diagnostic and are owned by subsequent phase-1
steps (1.6 primitive returns; 1.7..1.9 struct shapes; 1.10 multi-
keyword selectors).

Selector resolution is still per-call-site `sel_registerName` —
the planned 1.5 interning turns the per-call hashtable lookup into
a single static-global load. Chess Android + iOS-sim builds clean
— no regression on the existing typed-`objc_msgSend`-cast pattern.

tests/expected/ffi-objc-call-02-void-return.exit

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

tests/expected/ffi-objc-call-02-void-return.txt

@@ -1 +1 @@
-/Users/agra/projects/sx/examples/ffi-objc-call-02-void-return.sx:18:9: error: unresolved: 'unknown_expr'
+ok