fix: protocol method calls arity-check (issue 0131)

emitProtocolDispatch now requires the user-arg count to equal the
protocol method's parameter list — exact, since protocol signatures
have no defaults, packs, or variadics — and emits the same
"expects N arguments, but M were given" diagnostic plain calls get.
Previously extra args were silently dropped (and missing args left the
thunk reading garbage). The dispatch gains the call-site span for the
diagnostic. examples/1634 pins the rejection; full sweep confirms no
existing code relied on the leniency.

src/ir/lower/call.zig

@@ -861,7 +861,7 @@ pub fn lowerCall(self: *Lowering, c_in: *const ast.Call) Ref {
             // protocol value as its first param).
             if (self.getProtocolInfo(obj_ty)) |proto_info| {
                 if (protocolHasMethod(proto_info, fa.field)) {
-                    return self.emitProtocolDispatch(obj, proto_info, fa.field, args.items, obj_ty);
+                    return self.emitProtocolDispatch(obj, proto_info, fa.field, args.items, obj_ty, c.callee.span);
                 }
             }
 
@@ -878,7 +878,7 @@ pub fn lowerCall(self: *Lowering, c_in: *const ast.Call) Ref {
                     const pay_ty = opt_info.optional.child;
                     if (self.getProtocolInfo(pay_ty)) |proto_info| {
                         if (protocolHasMethod(proto_info, fa.field)) {
-                            return self.emitProtocolDispatch(obj, proto_info, fa.field, args.items, pay_ty);
+                            return self.emitProtocolDispatch(obj, proto_info, fa.field, args.items, pay_ty, c.callee.span);
                         }
                     }
                 }