ffi M1.0 (3/3): accept '=>' body in '#objc_class' member methods

Extends parseForeignClassDecl ([src/parser.zig:1262]) with an
arrow arm that mirrors the existing parseFnDecl shape — single-
expression body wrapped in a one-statement block so downstream
lowering sees the same AST as a brace-body method.

Closes the M1.0 surface: '=> expr;' is now valid for top-level
functions, struct methods, AND '#objc_class' member methods.
The sx-defined class lowering (A.7 dispatch gate) is still gated,
so 140-expression-bodied-objc-method.sx exercises parse-only —
the body is reachable but the method is never invoked. When M1.2
lights up sx-defined class instantiation, the arrow-body form
will flow through unchanged.

169 examples pass (+1 from 140 now green); zig build test green.

tests/expected/140-expression-bodied-objc-method.exit

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

tests/expected/140-expression-bodied-objc-method.txt

@@ -1 +1 @@
-/Users/agra/projects/sx/examples/140-expression-bodied-objc-method.sx:15:35: error: expected ';'
+