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).

library/modules/std/objc.sx

@@ -1,9 +1,5 @@
 // Obj-C runtime FFI primitives.
 //
-// `*void` stands in for the Obj-C `id`/`Class`/`SEL` types. There's no
-// sx-level type alias yet, so naming discipline at call sites is the only
-// thing keeping them apart.
-//
 // objc_msgSend has the standard ARM64 calling convention (no varargs path).
 // Each call site must invoke through a function pointer of the *exact*
 // argument and return shape. The idiom:
@@ -11,6 +7,28 @@
 //     msg_fn : (recv: *void, sel: *void, arg: [*]u8) -> *void = xx objc_msgSend;
 //     result := msg_fn(receiver, selector, c_string);
 
+// ─── Obj-C primitive type aliases ───────────────────────────────────────
+// Named stand-ins for the three opaque Obj-C runtime types. They all
+// resolve to `*void` at the LLVM layer (no runtime cost) but improve
+// readability in foreign-class declarations and call sites.
+//
+//   id    — any Obj-C instance pointer
+//   Class — a class object pointer
+//   SEL   — a registered selector
+//
+// `Class(T)` parameterization (phantom-typed, with `#extends`-aware
+// covariance) is a follow-up — needs compiler-level type-check support.
+// For now, `Class` alone is the only form; assignments are not checked
+// against the referent's class hierarchy.
+id    :: *void;
+Class :: *void;
+SEL   :: *void;
+
+// Apple's `BOOL` is a signed char (NOT sx's built-in `bool`, which is
+// LLVM `i1`). Obj-C method signatures that take or return `BOOL` cross
+// the FFI boundary as `s8`.
+BOOL :: s8;
+
 // On macOS libobjc is auto-loaded by libSystem; on iOS it isn't, so we
 // link it explicitly. Foundation registers NSString etc. with the runtime,
 // also auto-loaded on macOS and required as an explicit framework on iOS.