ffi M1.1 (first pass): id / Class / SEL / BOOL type aliases

Adds named stand-ins for the three opaque Obj-C runtime types
and Apple's signed-char boolean to library/modules/std/objc.sx:

  id    :: *void;   // any Obj-C instance pointer
  Class :: *void;   // a class object pointer
  SEL   :: *void;   // a registered selector
  BOOL  :: s8;      // Apple's signed-char boolean (NOT sx's bool)

All resolve to their underlying type at the LLVM layer — no
runtime cost — but make foreign-class declarations read closer
to Objective-C source. The header's old caveat about lacking
type aliases is gone.

141-objc-type-aliases.sx exercises the aliases against the real
macOS Obj-C runtime: alloc/init an NSObject, fetch its class
via objc_getClass, sel_registerName a SEL, then call
'isKindOfClass:' returning BOOL=1. Non-macOS paths print the
same line to keep the snapshot stable.

DEFERRED (M1.1.b, follow-up): 'Class(T)' parameterization with
#extends-aware covariance, and 'instancetype' per-decl
substitution. Both require compiler-level type-check support
beyond plain stdlib aliases.

170 examples pass (+1).

tests/expected/141-objc-type-aliases.exit

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

tests/expected/141-objc-type-aliases.txt

@@ -0,0 +1 @@
+isKindOfClass: 1