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.
parseForeignClassDecl ([src/parser.zig:1262]) accepts ';'
(declaration) or '{ ... }' (block body) but not '=>' for member
methods. The arrow form, which parseFnDecl ([src/parser.zig:1647])
already handles for top-level/struct decls (M1.0 1/3), surfaces
'expected ;' at the arrow today.
Snapshot pins that error so the next commit (the parser
extension) shows up as a single diagnostic→runtime-output diff
in 140-expression-bodied-objc-method.{txt,exit}.
