ffi M2.1(a): class-level constants 'name :: Type = expr;'

Inside a '#objc_class { ... }' block, 'name :: Type = expr;' is
accepted alongside the existing method form. Parsed as sugar for
'name :: () -> Type => expr;' — a niladic class method with an
expression body. The synthesized class method flows through the
M2.1(b) class-method pipeline: a C-ABI IMP is emitted and
registered on the metaclass.

Apple's runtime sees zero distinction — '[Cls foo]' dispatches to
our IMP regardless of source spelling. The constant form is
purely syntactic sugar; it reads better for static metadata
returns:

  SxGLView :: #objc_class("SxGLView") {
      layerClass :: Class = CAEAGLLayer.class();
  }

vs. the equivalent method form:

  layerClass :: () -> Class => CAEAGLLayer.class();

Parser change: after 'name ::' if the next token isn't '(' we
take the constant branch — parse a type expr, expect '=', parse
the value expr, expect ';'. The result is a ForeignMethodDecl
with is_static=true, empty params, return_type=Type, body=block
wrapping the expr. Pure parser-level transformation; no new AST
nodes, no new lowering passes.

150-objc-class-level-constant.sx exercises both shapes on macOS:
a primitive (s32 answer) and a pointer ('*NSObject seedClass'
— the canonical '+layerClass'-style factory return).

180 example tests pass (+1). zig build test green.

M2.1 complete: both (a) the constant form and (b) the
expression-bodied class method shape land.

Next: M2.2 — 'field: T #property(modifiers...)' synthesizes
getter/setter pairs.

tests/expected/150-objc-class-level-constant.exit

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

tests/expected/150-objc-class-level-constant.txt

@@ -0,0 +1 @@
+class constants: answer=42, seedClass=ok