Adds a special case to lowerFieldAccess: when the field is
literally 'class' and the receiver is a pointer to an Obj-C
(or Obj-C protocol) foreign-class struct, emit
'object_getClass(obj)' instead of falling through to struct GEP.
Returns 'Class' (the M1.1 first-pass alias for *void;
parameterized Class(T) covariance is deferred to M1.1.b).
f := SxFoo.alloc();
cls := f.class; // → object_getClass(f)
cls == objc_getClass("SxFoo".ptr); // ok
New helper isObjcClassPointer(ty) detects 'ptr -> struct in
foreign_class_map under .objc_class / .objc_protocol'. The
check fires BEFORE the auto-deref so the runtime call sees the
opaque Obj-C pointer rather than the load'd struct stub.
148-objc-self-class-accessor.sx exercises both shapes end-to-end
against the macOS runtime: sx-defined class (SxFoo) and foreign
class (NSObject). Round-trips against objc_getClass(name).
178 example tests pass. zig build test green.
This effectively closes Month 1 — M1.0, M1.1 (first pass), M1.2,
M1.3 all done. Remaining: M1.1.b (Class(T) covariance +
instancetype), then Month 2 (declarative sugar).
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